Raman Spectroscopic Study of Ruddlesden—Popper Tetragonal Sr₂VO₄

Abstract

The lattice dynamics of tetragonal Sr₂VO₄ with a Ruddlesden—Popper-layered crystal structure was studied via Raman spectroscopy. We observed three of the four expected Raman-active modes under ambient conditions. Mode Grüneisen parameters and the implicit fractions of two A_1g Raman-active modes were determined from high-pressure and high-temperature Raman spectroscopy experiments. The low-energy A_1g Raman-active mode involving Sr motions along the c direction has a large isothermal Grüneisen parameter about seven times larger than that of the high-energy A_1g Raman-active mode involving apical O motions along the c direction and is, therefore, more anharmonic. The thermodynamic Grüneisen parameter is significantly smaller in Sr₂VO₄ than in Sr₂TiO₄ due to the smaller Grüneisen parameter of the high-energy A_1g Raman-active mode and other vibrational modes that still need to be identified. The explicit contribution of the low-energy A_1g Raman-active mode is negative, and the implicit contribution due to volume change is much larger. Both volume implicit and anharmonic explicit contributions of the high-energy A_1g Raman-active mode have similar positive values. The Raman experiment in the air shows that Sr₂VO₄ begins to decompose above 200 °C.

1. Introduction

Ruddlesden—Popper (RP) A_n+1MnO_3n+1 (where A is an alkaline, alkaline-earth or rare-earth metal and M is a transition metal) compounds have layered crystal structures derived from the stacking of perovskite AMO₃-structured and AO-rock–salt-structured layers to form a natural superlattice [1,2]. They crystallize in a body-centered tetragonal structure with an I4/mmm space group [1,2]. The most prominent compounds with in the Ruddlesden—Popper phase are high-temperature superconducting cuprates [(La,A)₂CuO₄ (A = alkaline earth)] found in the mid-1980s [3,4]. In this case, the Ruddlesden—Popper (RP) structure with n = 1 corresponds to the K₂NiF₄ structure, whereas the perovskite structure corresponds to n = ∞ [1,2]. Another noteworthy class of RP compounds is formed by rare-earth nickelates A_n+1Ni_nO_3n+1, which have been studied as materials for solid oxide fuel cells (SOFC) owing to the high mobility of oxygen atoms in this structure due to interstitial oxygen atoms when n = 1 (in the case of R₂NiO_4+δ with R = La, Pr or Nd) or oxygen vacancies when n = 3 (in the case of R₄Ni₃O_10-δ with R = La, Pr or Nd) [5,6]. Generally, the tetragonal crystal structure is distorted in these nickelates and can only be recovered at high temperatures in certain cases, such as Pr₂NiO_4+δ [5,7]. Ruddlesden—Popper compounds within the K₂NiF₄ structure display a rich variety of electronic ground states, giving rise to many interesting properties, such as high Tc and exotic superconductivity, metal—insulator transitions and various magnetic and spin—orbital states [3,4,8,9,10,11,12,13,14,15,16,17,18,19,20], which are related to its two-dimensional nature. In particular, Ruddlesden—Popper Sr₂MO₄ compounds with a K₂NiF₄ tetragonal structure and d-electrons around the Fermi level have various complex and correlated properties [8,9,10,11,12,13,14,15,16,17,18,19,20]. Several of these compounds are Mott insulators [8,9,10,11,12]. More interestingly, correlated superconductivity with spin—triplet pairing was discovered in Sr₂RuO₄ [18,19,20], and complex 2D spin—orbital ordering was found in Sr₂IrO₄ [12,13] and Sr₂VO₄ [16]. Among these materials, tetragonal Sr₂VO₄ has attracted attention firstly because it is a Mott—Hubbard insulator with a 3d¹ configuration, which is the electron counterpart of the parent compound of the high-Tc La₂CuO₄ superconductors with a 3d⁹ configuration [8,9,21,22,23]. Matsuno et al. found that a thin film of Sr₂VO₄ grown on a (100) LaSrAlO₄ substrate has a charge transfer gap of 2.8 eV and a Mott—Hubbard gap of about 0.2 eV [8,23]. However, Fukuda et al. also grew thin films of Sr₂VO₄ on (100) LaAlO₃ and on (100) LaSrGaO₄ substrates and found non-zero absorption down to about 0.1 eV in the first case, indicating a non-zero density of states at the Fermi level [17]. The discovery of possible orbital ordering in Sr₂VO₄ at about 100 K in 2007 made this material of great interest by itself [16]. However, this orbital ordering is very sensitive to oxygen stoichiometry and appears only in stoichiometric samples of good quality with smaller c lattice parameters and a semiconducting character [24,25]. Many experimental [16,24,25,26,27,28,29,30,31,32,33,34] and theoretical [22,33,35,36,37,38,39,40] studies have investigated the origin and nature of this orbital ordering. Inelastic neutron scattering experiments showed the presence of a magnetic doublet at about 120 meV, which was proposed to come from orbital liquid due to orbital fluctuations at high temperatures and from orbital ordering at low temperatures [26]. The signature of these collective excitations was also observed in infrared spectroscopy experiments, albeit with a different interpretation [27]. If there is no magnetic ordering associated with the transition at about 100 K, weak magnetic ordering was found below 8–10 K [24,27,28,30,32]. Heat capacity experiments [24,27,28], as well as diffraction experiments [16,32,33], showed the presence of two-phase transitions at about 100 K and at about 130 K and the occurrence of a weak orthorhombic distortion due to the Jahn—Teller effect in this temperature range [32,33]. This intermediate orthorhombic phase between 100 K and 130 K can be described in the Immm space group [32,33]. These phase transitions disappear with La doping [41,42] or hydrogenation [43,44]. Pressure-induced metal—insulator transitions have been found above 20 GPa by Karmakar and Malavi [31] but at lower pressure (6 GPa) by Yamauchi et al. on better samples [34]. Also, the intermediate orthorhombic phase disappears at this pressure, and the phase diagram of Sr₂VO₄ is complex with two different metallic phases as follows: above 150 K and below 40 K at pressures above 6 GPa [34]. Several models were proposed to describe the low T ground state of Sr₂VO₄, some of which were before the experimental clarification discussed above. Zhou et al. originally proposed an orbitally ordered ground state due to the Jahn—Teller distortion [16]. Imai et al. predicted that Sr₂VO₄ is close to the metal—insulator transition and that its ground state is non-trivial and orbital-stripe-ordered, with a complex pattern of long periodicity [35,36]. Jackeli and Khaliullin proposed a magnetically hidden octupolar order induced via spin—orbit coupling in Sr₂VO₄ [37]. Eremin et al. proposed an alternating spin—orbital order with a muted magnetic moment, where the spin and orbital moments canceled each other [38]. In order to explain the intermediate orthorhombic phase, Teyssier et al. proposed that superexchange and spin—orbit coupling yield an antiferro—orbital ground state compete with nematic orbital ordering due to a dynamic Jahn—Teller effect, which is screened below 100 K where the tetragonal structure is recovered [33]. Kim et al. found that a short-range stripe order causes the tetragonal-to-orthorhombic transition at 130 K but no magnetic transition because the stripe order is frustrated [39]. At lower temperatures, competition with other magnetic phases, such as the double stripe order along (100), can destroy the short-range stripe order, and, at the lowest temperatures, some liquid-spin or glass-spin states appear with strong short-range ferromagnetic interactions [39]. These different ground states proposed by the various models above often involve some spin—lattice coupling, which has a major impact on the lattice dynamics and temperature variation in the phonon energies. Therefore, a good understanding of the lattice dynamics of Sr₂VO₄ could be helpful to understand its fundamental properties. The study of infrared-active modes as a function of temperature has shown a close connection between lattice dynamics and various transitions [27]. The ab initio calculations for the lattice dynamics of Sr₂VO₄ show that Raman-active modes have frequencies very close to Sr₂TiO₄, but this is not the case for the infrared-active modes [45]. Some of us have recently reported Raman-scattering experiments on Sr₂TiO₄ as a function of temperature and pressure [46], and it could be interesting to perform similar experiments on Sr₂VO₄ in order to see the possible effect of orbital fluctuations observed in inelastic neutron-scattering experiments [26]. Thus, in the present work, we report an experimental study in which the lattice dynamics in Sr₂VO₄ are measured via Raman spectroscopy as a function of pressure and temperature, permitting the determination of anharmonicity for Raman-active modes in Sr₂VO₄.

2. Experimental Details

Details of the preparation of Sr₂VO₄ ceramic samples are described in our previous work [24,27]. These samples were obtained via the solid-state reaction of a stoichiometric mixture of 99.999% SrCO₃ and 99.999% V₂O₅ in two steps. Firstly, Sr₄V₂O₉ was obtained after ball milling and calcination at 800 °C for 60 h in the air. Secondly, after ball milling, Sr₄V₂O₉ was reduced at 900–950 °C for 48 h in evacuated quartz tubes with Zr as a reducing agent. This procedure was repeated a second time, and we obtained almost pure tetragonal Sr₂VO₄ based on X-ray diffraction (XRD), as in our previous work [24,27].

Magnetic susceptibility experiments were performed in a SQUID Magnetic Property Measurement System (MPMS) from Quantum Design at between 2 K and 300 K.

We performed Raman-scattering experiments with a Horiba Jobin Yvon Labram Aramis spectrometer equipped with a Peltier-cooled CCD. We used a blue diode laser with a wavelength of 473 nm. High-temperature Raman experiments were performed between room temperature and 400 °C using an oven with a Linkham TS1500 heating stage under the objective lens (×50) of an Olympus microscope. A Pt resistance thermometer located at the bottom of the oven was used for temperature measurements.

High-pressure Raman spectroscopy experiments were performed between room pressure and about 6 GPa within a membrane diamond anvil cell (DAC) with low fluorescence diamonds. The samples were loaded into a chamber drilled in an indented stainless-steel gasket. We used glycerol as a pressure-transmitting medium. Because it maintains hydrostatic conditions up to 1.4 GPa [47], our experiments were limited to about 6 GPa in order to avoid excessive non-hydrostatic effects. The pressure was determined from the fluorescence of ruby, which is a well-known pressure gauge [48].

3. Results and Discussion

In Figure 1, we show the magnetic susceptibility of our Sr₂VO₄ sample. There is a broad maximum at about 103 K corresponding to the orbital ordering and a sharp maximum at 10 K, below which there is a hysteresis between zero-field-cooling and field-cooling measurements due to unknown weak magnetic ordering [24,27,28,30,32]. According to previous studies [24,25], the presence of this transition in our sample means that it is near the stoichiometry of Sr₂VO₄. These observations demonstrate the good quality of our sample. After one year, there was no significant change, which is in agreement with our previous work on another sample [24], but after two years, different magnetic susceptibility features at about 10 K and 103 K disappeared. According to Ueno et al. [25], this change in magnetic susceptibility meant that the oxygen content decreased and, therefore, some oxygen vacancies appeared over time because oxygen should be mobile in Sr₂VO₄ at ambient conditions. Additional experiments are needed to confirm this point and study the kinetic effects.

The primitive unit cell of Sr₂VO₄ contains seven atoms, meaning there are 21 vibrational modes. Among them, there are three acoustic modes and eighteen optical modes. The symmetries of optical modes could be obtained from their decomposition into irreducible representations [46]:

Γ = 2 A_1g ⊕ 2 E_g ⊕ 3 A_2u ⊕ 4 E_u ⊕ B_2u

One can then obtain selection rules for Raman spectroscopy with four Raman-active modes (2 A_1g ⊕ 2 E_g) and, for infrared spectroscopy, using seven infrared active modes (3 A_2u ⊕ 4 E_u). There is one silent B_2u mode [46].

We report Raman spectra as functions of temperature and pressure in Figure 2. We could see one broad and intense line at 555.6 cm⁻¹, one broad and rather weak peak at 196 cm⁻¹, and a very tiny and reproducible feature at about 288 cm⁻¹. The features at about 350 cm⁻¹ in the spectra at P = 0.71 GPa and T = 300 °C should correspond to an oxygen-rich secondary phase (see the Section 3 below). Our results agree reasonably well with the DFT calculations of Kong et al. [45], and one could attribute the two most intense lines to A_1g modes, whereas the tiny feature was an E_g mode. The frequencies of these different modes were rather close to those of Sr₂TiO₄ [46]. This is not surprising because Raman-active modes involve only Sr and apical O atoms of the VO₆ octahedra [45,46,49], and the lattice parameters of Sr₂VO₄ are only slightly smaller than those of Sr₂TiO₄. The low-energy A_1g Raman-active mode involves Sr motions, whereas the high-energy A_1g Raman-active mode involves apical O motions, which in both cases are along the c direction [45]. The E_g Raman-active mode observed at about 288 cm⁻¹ involves apical O motions in the ab plane. Using a constant force model, Saini et al. were able to find which contributions to force constants were dominant for different vibrational modes of tetragonal Sr₂MO₄ compounds (M = Ti, V and Mn) [50]. For the high-energy A_1g mode, although high frequencies are primarily due to VO₆ vibrations, they not only found that stretching force constants between apical O atoms and V atoms were dominant but also that bending forces between apical O atoms and Sr atoms were as well [50]. The two main contributions to the low-energy A_1g mode come from two bending forces: those between the equatorial O atoms of the VO₆ octahedra and Sr atoms and those between Sr atoms and V atoms [50]. In the case of the E_g Raman-active mode observed at about 288 cm⁻¹, this involves stretching-force constants between apical O atoms and V atoms [50]. In a previous study, all seven infrared-active modes of Sr₂VO₄ were found, and their dependence on temperature was studied below room temperature [27]. The four infrared active modes with wavenumbers lower than 400 cm⁻¹ showed an unusual softening with a decreasing temperature above the orbital ordering temperature, probably related to the low-temperature phase transition [27]. We did not see such softening for the Raman-active modes.

Within the isotropic Grüneisen approximation, the isothermal Grüneisen parameter ${\gamma }_i^T$ and the isobaric Grüneisen parameter ${\gamma }_i^P$ of the ith mode of ω_i frequency can be defined as follows [51,52]:

$${\gamma }_i^T=B{\left(\frac{\partial ln{\omega }_i}{\partial P}\right)}_T \mathrm{and} {\gamma }_i^P=\frac{-1}{{\alpha }_V}{\left(\frac{\partial ln{\omega }_i}{\partial T}\right)}_P$$

(1)

where B is the bulk modulus and α_V is the volume thermal expansion.

The expression of the isobaric Grüneisen parameter is, therefore [52],

$${\gamma }_i^P={\gamma }_i^T+{\gamma }_i^V$$

(2)

Thus, isobaric Grüneisen parameters ${\gamma }_i^P$, which are determined from temperature-dependent Raman-scattering experiments, had the following two different contributions: ${\gamma }_i^T$ and ${\gamma }_i^V$. The first term is the isothermal Grüneisen parameter ${\gamma }_i^T$ from the volume-driven contribution corresponding to the implicit effect, whereas the second term is the isochoric Grüneisen parameter ${\gamma }_i^V$ from the occupation number-driven or amplitude-driven contribution corresponding to the explicit effect, which is a measure of the anharmonic contribution [52]. From high-pressure Raman-scattering experiments, isothermal Grüneisen parameters ${\gamma }_i^T$ could be obtained. The magnitude of the implicit and explicit contributions to each mode could be determined via the implicit fraction η_i = ${\gamma }_i^T$/${\gamma }_i^P$.

In Figure 3, we report temperature and pressure variations of the frequencies of both A_1g modes. The pressure variation of the frequencies of these modes was more or less linear at 3.46 cm⁻¹/GPa for the low-frequency mode and 1.44 cm⁻¹/GPa for the high-frequency mode. This is the same magnitude as the Sr₂TiO₄ results for the low-frequency A_1g mode but is much smaller than that of Sr₂TiO₄ for the high-frequency A_1g mode [46,53]. In order to determine the isothermal mode Grüneisen parameters ${\gamma }_i^T$, one needs to know the bulk modulus. Yamauchi et al. determined the pressure dependence of volume in Sr₂VO₄ [34]. One can fit Yamauchi’s data [34] to the Vinet equation of states [54,55], and this yields B = 135 ± 15 GPa with B’ = 4. This is smaller than the bulk modulus obtained from DFT calculations for Sr₂TiO₄ [46]. If we use the bulk modulus obtained from Yamauchi’s data [34], one obtains ${\gamma }_i^{T }$= 2.38 for the low-energy A_1g mode and ${\gamma }_i^{T }$= 0.35 for the high-energy A_1g mode. The Grüneisen parameter ${\gamma }_i^T$ of the low-energy A_1g mode of Sr₂VO₄ is very close to that of Sr₂TiO₄ [46] and seven times larger than that of the high-energy A_1g mode of Sr₂VO₄. The Grüneisen parameter ${\gamma }_i^T$ for the high-energy A_1g mode of Sr₂VO₄ is about three times smaller than that of Sr₂TiO₄ [46].

In Figure 3, one can see that the frequencies of both A_1g Raman-active modes decrease with increasing temperature, i.e., the standard behavior. The behavior of these two A_1g Raman-active modes is similar above room temperature in Sr₂TiO₄ [46]. This increase was somewhat linear for the high-energy A_1g mode but not really for the low-energy A_1g mode. Nevertheless, we performed the linear fitting of data between room temperature and 300 °C and found −0.011 cm⁻¹/K and −0.012 cm⁻¹/K for the low-energy and high-energy A_1g modes, respectively. We could determine the isobaric Grüneisen parameters ${\gamma }_i^P$ for these modes using the thermal expansion coefficient, which could be obtained from the thermal variation in lattice parameters in Sr₂VO₄ from Teyssier et al. between 200 K and 280 K [33]. Contrary to the case of Sr₂TiO₄, where thermal expansion was almost isotropic, thermal expansion was much larger in the ab plane (α_a = 13 MK⁻¹) rather than in the c direction (α_c = 5.1 MK⁻¹). The latter value, as well as the volume of thermal expansion (α_V = 31.1 MK⁻¹), was smaller in Sr₂VO₄ than in Sr₂TiO₄. Using the volume thermal expansion data given above, we obtained ${\gamma }_i^P$= 1.82 for the low-energy A_1g mode and ${\gamma }_i^P$= 0.69 for the high-energy A_1g mode. The ${\gamma }_i^P$ values for both A_1g modes were slightly larger than those determined for Sr₂TiO₄ [46]. The explicit anharmonic contribution to the low-energy A_1g mode had a small and negative value (${\gamma }_i^V$ = −0.56), whereas it was small and positive for the high-energy A_1g mode (${\gamma }_i^V$ = 0.34). The ${\gamma }_i^V$ value for the low-energy A_1g mode of Sr₂VO₄ was similar to that of Sr₂TiO₄. In the case of the high-energy A_1g mode of Sr₂TiO₄, ${\gamma }_i^V$ was also negative [46], contrasting to the present case of Sr₂VO₄. This is because the volume contribution of ${\gamma }_i^T$ was much smaller for the high-energy A_1g mode of Sr₂VO₄. Thus, when calculating the implicit ratio, we obtained η_i = 1.3 for the low-energy A_1g mode and η_i = 0.5 for the high-energy A_1g mode for the case of Sr₂VO₄. The ratio η_i of the high energy A_1g mode was greater than the 0.5 determined for the high-energy A_1g mode; as this was also the case for different Raman-active modes of Sr₂TiO₄, it means that implicit volume is the dominant contribution to temperature dependence. For the high-energy mode, the ratio η_i was about 0.5, meaning that both implicit and explicit contributions contributed equally. Clearly, in Sr₂VO₄, the anharmonicity of the low-energy A_1g mode was much larger than that of the high-energy A_1g mode, where anharmonicity was rather weak, particularly in comparison with Sr₂TiO₄.

We can now determine the thermodynamic Grüneisen parameter Γ from the following expression [56]:

$$\Gamma =\frac{{\alpha }_{V}BV}{C_P}$$

using the volume thermal expansion α_V (31.1 MK⁻¹) determined from the thermal variation of the lattice parameters of Sr₂VO₄ and the volume of the primitive unit cell at 280 K (92.327 ų) measured by Teyssier et al. [33], the bulk modulus B (135 GPa) determined from the pressure dependence of the lattice parameters of Sr₂VO₄ measured by Yamauchi et al. [34] and the heat capacity C_P (157.5 J/mole.K) of Sr₂VO₄ at 280.4 K [57].

This yielded Γ = 1.48 for Sr₂VO₄. This value is midway between the Grüneisen parameters of ${\gamma }_i^T$ for the low-energy and high-energy A_1g modes. This is smaller than the value of about two found in Sr₂TiO₄ in our previous work [46]. This smaller value could indicate not only that the high-energy A_1g mode has a lower Grüneisen parameter of ${\gamma }_i^T$ but that this is probably the case for some other modes as well. The study of infrared active modes under high pressure could help to identify other vibrational modes for which the Grüneisen parameters ${\gamma }_i^T$ are lower. It is important to note that this is mainly related to thermal expansion along the c direction, which was significantly smaller in Sr₂VO₄ than in Sr₂TiO₄, and the high-energy A_1g Raman-active mode, for which the smaller Grüneisen parameter of ${\gamma }_i^T$ involves apical O motions along the c direction. We expect, therefore, that if there are some other modes with a small Grüneisen parameter for ${\gamma }_i^T$, they must involve vibrations along the c direction as well.

The full-width at half maximum (FWHM) Δ_i for the Raman modes can be expressed as a function of the residual term Δ_i(0) and the 3-phonon interaction term in first approximation as follows [51,52,58]:

$${\Delta }_{\mathrm{i}}={\Delta }_{\mathrm{i}}\left(0\right)+A\left(1+\frac{2}{e^x-1}\right)$$

(3)

where the term Δ_i(0) is constant and x = ħω_i/2k_BT.

The FWHM Δ_i of the high-energy A_1g mode can be modeled using Equation (3) in Figure 4, where Δ_i(0) = 22 cm⁻¹ and A = 21 cm⁻¹. The large constant term Δ_i(0) was due to a large number of defects in the O sublattice, and this temperature-dependent term was also quite large, indicating significant anharmonicity. Both terms were significantly larger in Sr₂VO₄ than in Sr₂TiO₄, for which the FWHM Δ_i of the high-energy A_1g mode was also quite large (between 35 and 55 cm⁻¹ for 300 K < T < 600 K). This indicates more defects and greater anharmonicity in Sr₂VO₄ than in Sr₂TiO₄. However, when analyzing the pressure and temperature dependence of the peak position for the high-energy A_1g mode, we observed that the anharmonicity of this mode was weaker in Sr₂VO₄ compared to Sr₂TiO₄. To reconcile these contradictory results, we suggest that the reason why FWMH is larger in Sr₂VO₄ than in Sr₂TiO₄ must be mainly due to the amount of defects, which should increase with the temperature. We also want to mention that, in the case of RP nickelates, the apical M—O bond was positioned in competing rock—salt and perovskite motifs of the structure, which may have resulted in the more anharmonic apical O motions along the c direction in relation to the large thermal displacements of apical oxygen [6,59]. The difference between this and the results for tetragonal Sr₂VO₄ is not yet understood.

Finally, we want to note that the Raman experiments at high temperature reported in Figure 3 were performed under an Ar atmosphere and that the small features appearing at about 300–350 cm⁻¹ increased significantly above 400 °C and are due to oxygen contamination of the sample that oxidizes Sr₂VO₄. This observation was confirmed via other Raman experiments under the air in which these features began to appear above 200 °C, and the signal of Sr₂VO₄ completely disappeared at 400 °C. This meant that Sr₂VO₄ was stable up to 200 °C under the air. This is a higher temperature than the 127 °C reported by Ueno et al. [25]. The higher decomposition temperature we observed could be related to the decomposition kinetics of tetragonal Sr₂VO₄. Indeed, the duration of the measurement of one Raman spectrum was 10 min, which is much shorter than the measurement of usual powder patterns using laboratory diffractometers. In addition, it might have been sensitive to inhomogeneities in the sample; micro-Raman spectroscopy is a local probe, whereas powder XRD was averaged on the sample.

4. Conclusions

The vibrational properties of the Ruddlesden—Popper-layered compound Sr₂VO₄ were studied via Raman spectroscopy as a function of pressure and temperature. We observed three of the four Raman-active-expected modes and determined mode Grüneisen parameters and the implicit fractions of these two A_1g Raman-active modes. The low-energy A_1g Raman-active mode had a large isothermal Grüneisen parameter (2.38) seven times larger than the isothermal Grüneisen parameter of the low-energy A_1g Raman-active mode. The thermodynamic Grüneisen parameter of Sr₂VO₄ had an intermediate value of 1.48, which is lower than the value for Sr₂TiO₄ [46]. This is related to the isothermal Grüneisen parameter of the low-energy A_1g Raman-active mode, which is much smaller in Sr₂VO₄ than in Sr₂TiO₄ [46], as well as some other vibrational modes that are yet to be identified. As in Sr₂TiO₄ [46], the low-energy A_1g Raman-active mode involving Sr motions along the c direction was much more anharmonic than the high-energy A_1g Raman-active mode involving apical O motions along the c direction. The low-energy A_1g Raman-active mode has a negative explicit contribution and a much larger implicit volume contribution, whereas the high-energy A_1g Raman-active mode had similar positive contributions from both the implicit volume implicit contribution and the explicit anharmonic contribution. Raman experiments in the air show that Sr₂VO₄ begins to decompose above 200 °C.