**1. Introduction**

Recently, luminescent materials have been the focus of interest of the research community due to their vast suitability for applications, namely, display, solar cells, biomedical, sensing, etc. [1–3]. The benefits of these luminous materials in a variety of applications have prompted scientists to look for new and better materials with enhanced luminescence capabilities [4]. This artificial luminescence is used for plant growth and development in greenhouses [5]. The lighting conditions in plant growth are directly related to the success of the production. Plant lighting demand and energy usage are growing in tandem with the advancement of modern agriculture. Generally, in plant production systems, traditional light sources such as fluorescent lamps, incandescent lamps, and high-pressure sodium lamps are used [6]. However, traditional light sources were primarily designed to support human activities based on the sensitivity of the human eye rather than the absorption spectra of plants. This type of traditional lamp for crop systems suffers from high energy consumption and a serious spectral mismatch between its emitting spectra and the absorption spectra of plants. Hence, light-emitting diodes (LEDs) have become an unavoidable choice in comparison to traditional light sources due to their quick response time, long lifetimes, energy savings, low cost, and reliability [7–9]. LED wavelengths can be modified using various phosphors to fit the spectral range of plant photosynthesis and photo morphogenesis, potentially affecting plant growth and development through modulating phytochrome. It is known that light impacts various developmental processes in plants, including seed germination, blooming, fruiting, and other morphogenesis, in addition to being a vital energy source for photosynthesis [10,11]. Blue (400–500 nm), red

**Citation:** Bharat, L.K.; Patnam, H.; Sokolov, A.; Gudkov, S.V.; Yu, J.S. Red Light Emitting Transition Metal Ion Doped Calcium Antimony Oxide for Plant Growth Lighting Applications. *Agriculture* **2022**, *12*, 2066. https:// doi.org/10.3390/agriculture12122066

Academic Editor: Athanasios Koukounaras

Received: 20 October 2022 Accepted: 25 November 2022 Published: 1 December 2022

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(620–690 nm), and far-red (700–740 nm) lights are responsible for photosynthesis, phototropism, and photo morphogenesis, respectively [12]. It is important to note that plants' photoreceptor systems are particularly sensitive to red and far-red light, which are involved in the entire growth process [13,14]. As a result, the spectrum required for plant growth must correspond to that of photosensitive pigments.

Typically, Eu3+- and Eu2+-doped phosphors were reported for red emission. Eu3+-doped materials show sharp and narrow peaks with peak maxima in the wavelength range of 610 to 620 nm, which is considerably far from deep to far-red emission [15,16]. Similarly, Eu2+doped nitride phosphors are also confined due to precise and rigorous synthesis conditions and the high cost of rare-earths [17,18]. To overcome difficulties, transition metal ions such as Cr3+, Mn4+ can be chosen as alternative dopants for plant growth applications [19–21]. In this work we studied the optical properties of Mn4+doped CaSb2O6 materials. Mn4+ with a 3d3 electronic configuration generally stabilizes in an octahedron environment in most of the host materials. The optical properties of Mn4+ ions were significantly affected by the crystal field which is stronger in oxide- than in fluoride-based host materials [22]. The Mn4+ doped host materials show a broad excitation band due to the O2−-Mn4+ charge transfer and the 4A2g → 4T1g, 4A2g → 2T2g, and 4A2g → 4T2g transitions. Likewise, the emission is seen in the red region (550–800 nm) due to the 2Eg → 4A2g transition of the Mn4+ ion [23]. On this account, we can say that the similarity of the red or far-red emission of the Mn4+ doped oxide host materials and the desired spectral range of plants make the Mn4+ doped oxide materials an ideal choice for the LED application of plant growth.

In this context, we prepared an Mn4+doped CaSb2O6 (CSO: Mn4+) sample using the conventional high-temperature solid-state method. The sample was characterized to study the crystal structure, morphology, elemental composition, and luminescence properties. The emission of the CSO: Mn4+ sample was further compared with the absorption spectra of the photosensitive pigments of the plant to portray their use for plant growth LED application. Furthermore, the effect of the charge compensator on the luminescence properties was also studied.

#### **2. Materials and Methods**
