**Nanocrystalline Antiferromagnetic High-**κ **Dielectric Sr2NiMO<sup>6</sup> (M** = **Te, W) with Double Perovskite Structure Type**

**Jelena Bijeli´c <sup>1</sup> , Dalibor Tatar <sup>1</sup> , Sugato Hajra <sup>2</sup> , Manisha Sahu <sup>2</sup> , Sang Jae Kim <sup>2</sup> , Zvonko Jagliˇci´c 3,4 and Igor Djerdj 1,\***


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Academic Editor: Giuseppe Cirillo

Received: 7 August 2020; Accepted: 1 September 2020; Published: 2 September 2020

**Abstract:** Double perovskites have been extensively studied in materials chemistry due to their excellent properties and novel features attributed to the coexistence of ferro/ferri/antiferro-magnetic ground state and semiconductor band gap within the same material. Double perovskites with Sr2NiMO<sup>6</sup> (M = Te, W) structure type have been synthesized using simple, non-toxic and costless aqueous citrate sol-gel route. The reaction yielded phase-pure nanocrystalline powders of two compounds: Sr2NiWO<sup>6</sup> (SNWO) and Sr2NiTeO<sup>6</sup> (SNTO). According to the Rietveld refinement of powder X-ray diffraction data at room temperature, Sr2NiWO<sup>6</sup> is tetragonal (*I*4/*m*) and Sr2NiTeO<sup>6</sup> is monoclinic (*C*12/*m*1), with average crystallite sizes of 49 and 77 nm, respectively. Structural studies have been additionally performed by Raman spectroscopy revealing optical phonons typical for vibrations of Te6+/W6+O<sup>6</sup> octahedra. Both SNTO and SNWO possess high values of dielectric constants (341 and 308, respectively) with low dielectric loss (0.06 for SNWO) at a frequency of 1 kHz. These values decrease exponentially with the increase of frequency to 1000 kHz, with the dielectric constant being around 260 for both compounds and dielectric loss being 0.01 for SNWO and 0.04 for SNTO. The Nyquist plot for both samples confirms the non-Debye type of relaxation behavior and the dominance of shorter-range movement of charge carriers. Magnetic studies of both compounds revealed antiferromagnetic behavior, with Néel temperature (TN) being 57 K for SNWO and 35 K for SNTO.

**Keywords:** antiferromagnet; double perovskite; high-κ dielectric; nickel; tellurium; tungsten
