High temperature induced phase transitions in SrCaCoTeO6 and SrCaNiTeO6 ordered double perovskites

Crystallography at non-ambient conditions and physical properties of the synthesized double perovskites, Sr2(Co1-xFex)TeO6

Polycrystalline double perovskite-type Sr₂(Co_1-xFe_x)TeO₆ with various stoichiometric compositions (x = 0, 0.25, 0.5, 0.75, and 1) were synthesized by solid-state reactions in air. The crystal structures and phase transitions of this series at different temperature intervals were determined by X-ray powder diffraction, and from the obtained data the crystal structures were refined. It has been proven that for the compositions x = 0.25, 0.50, and 0.75, the phases crystallize at room temperature in the monoclinic space group I2/m. Down to 100 K, depending on the composition, these structures experience a phase transition from I2/m to P2₁/n. At high temperatures up to 1100 K their crystal structures show two further phase transitions. The first one is a first-order phase transition, from monoclinic I2/m to tetragonal I4/m, followed by a second-order phase transition to cubic Fm3̄m. Therefore, the phase transition sequence of this series detected at temperatures ranging from 100 K to 1100 K is P2₁/n → I2/m → I4/m → Fm3̄m. The temperature-dependent vibrational features of the octahedral sites were investigated by Raman spectroscopy, which furthermore complements the XRD results. A decrease in the phase-transition temperature with increasing iron content has been observed for these compounds. This fact is explained by the progressive diminishing of the distortion of the double-perovskite structure in this series. Using room-temperature Mössbauer spectroscopy, the presence of two iron sites is confirmed. The two different transition metal cations Co and Fe at the B sites allow exploring their effect on the optical band-gap.

Effects of iron substitution and anti-site disorder on crystal structures, vibrational, optical and magnetic properties of double perovskites Sr2(Fe1−xNix)TeO6

We report a new series of DP Sr2(Fe1−xNix)TeO6, which have different transition metal Fe and Ni on B sites, providing an opportunity to investigate their effect on crystal structure, vibrational, optical and magnetic properties.

Synthesis and Study of the Effect of Ba2+ Cations Substitution with Sr2+ Cations on Structural and Optical Properties of Ba2-xSrxZnWO₆ Double Perovskite Oxides (x = 0.00, 0.25, 0.50, 0.75, 1.00)

The effect of Sr²⁺ substitution on the morphology, crystal structure, and optical properties of double perovskite oxide Ba_2−xSr_xZnWO₆ (x = 0.00, 0.25, 0.50, 0.75, 1.00) were investigated. Scanning electronic microscopy demonstrated that all samples have similar microstructure morphology but differ in the range of grain sizes. X-ray diffraction measurements indicated that these materials crystallize in a (Fm-3m) cubic crystal structure, and also confirmed the tolerance factor. Rietveld analysis revealed that the lattice parameter decreased from 8.11834 to 8.039361 Å when the substitution of Ba²⁺ with Sr²⁺ cations increased from zero to 100%. Fourier transform infrared (FTIR) and Raman spectroscopies displayed a symmetric stretching vibration of WO₆ octahedra at 825 cm⁻¹, and an anti-symmetric stretching mode of WO₆ was observed by FTIR at 620 cm⁻¹. A strong peak at 420 cm⁻¹ was also observed in the Raman spectra and is due to the W–O–W bending vibration modes. UV-Vis diffuse reflectance spectroscopy was carried out for the series, and the band gap energy decreased from 3.27 eV for Ba₂ZnWO₆ to 3.02 and 3.06 eV for Ba_1.75Sr_0.25ZnWO₆ and Ba_1.5Sr_0.5ZnWO₆, respectively. The excitation and emission photoluminescence properties were investigated at room temperature.

The effect of partial substitution of Ni by Mg on the structural, magnetic and spectroscopic properties of the double perovskite Sr2NiTeO6

In this report, the structural, magnetic and spectroscopic properties of the freeze-drying synthesized Sr2Ni1-xMgxTeO6 (x = 0.0, 0.1, 0.2, 0.3 and 0.5) oxides are analyzed by means of X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD), electron paramagnetic resonance, diffuse reflectance and magnetic susceptibility. The XRPD and NPD data analysis using the mode-crystallography approach have revealed that at room temperature (RT), all the compositions are monoclinically distorted with the space group I2/m. The high and low temperature analyses have shown that these materials suffer a series of three structural phase transitions. The EPR results have shown that the spectra of all the compositions are centred at g≈ 2.28, indicating a slightly distorted octahedral environment of Ni(2+), which is in agreement with the crystal structure analysis. The increase of the Mg(2+) content in Sr2Ni1-xMgxTeO6, provokes a decrease of the dipolar interaction effects and thus, the resonance becomes narrower. This resonance does not completely disappear which leads to the idea that the long-range magnetic order is not completely established when x≥ 0.3. The substitution of the Ni(2+) (S = 1) ions by Mg(2+) (S = 0) ions, also induces a weakening of the antiferromagnetic interactions, which is reflected in the diminishing of the absolute value of θ and the Néel temperature TN. The magnetic structure determination revealed the existence of an antiferromagnetic coupling for x- and z-spin components of the nickel atoms.