Ramarao S D, Murthy V R K
Microwave Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India.
Phys Chem Chem Phys. 2015 May 21;17(19):12623-33. doi: 10.1039/c5cp00569h.
Temperature- and composition-induced phase transition in SmNbO4 was studied by differential scanning calorimetry, Raman spectroscopy and high-temperature powder X-ray diffraction measurements. In situ X-ray diffraction studies revealed that SmNbO4 possesses a monoclinic fergusonite crystal structure at ambient temperature and transforms to a tetragonal scheelite structure above the transition temperature (To ≥ 800 °C). The second-order nature of this transition was confirmed by observing a linear relationship between the spontaneous strain (es) of SmNbO4 and the Landau order parameter (η) around the phase transition temperature. We stabilized this high-temperature tetragonal scheelite phase at ambient temperature by substituting Si(4+) and Mo(6+) into the Nb site of SmNbO4. The SmNb1-x(Si1/2Mo1/2)xO4 (x = 0.0-0.69) ceramic compositions were prepared by the conventional solid-state reaction method. Rietveld refinement was carried out on all the compositions to examine the phase purity, and the compositions where x < 0.06 all formed a monoclinic fergusonite structure (I2/a space group, Z = 2). Both the X-ray diffraction and Raman spectroscopy measurements revealed that increasing the concentration of x transformed the structure from monoclinic fergusonite to tetragonal scheelite (I41/a space group, Z = 4) at a critical concentration (xc). Both the monoclinic and tetragonal phases coexisted in the composition range of 0.06 ≤ x < xc. The Hakki-Coleman and reflection cavity techniques were used to measure the dielectric constant and quality factor of these stabilized phases, respectively. The temperature coefficient of the resonant frequency was measured by using an invar cavity attached to a programmable hot plate. The high-density samples possessed good microwave dielectric properties.
通过差示扫描量热法、拉曼光谱和高温粉末X射线衍射测量研究了SmNbO₄中温度和成分诱导的相变。原位X射线衍射研究表明，SmNbO₄在室温下具有单斜铁钽矿晶体结构，并在转变温度（To≥800°C）以上转变为四方白钨矿结构。通过观察SmNbO₄的自发应变（es）与相变温度附近的朗道序参量（η）之间的线性关系，证实了这种转变的二级性质。我们通过将Si(4+)和Mo(6+)替代到SmNbO₄的Nb位点，在室温下稳定了这种高温四方白钨矿相。采用传统的固态反应法制备了SmNb₁₋ₓ(Si₁/₂Mo₁/₂)ₓO₄（x = 0.0 - 0.69）陶瓷组合物。对所有组合物进行了Rietveld精修以检查相纯度，x < 0.06的组合物均形成单斜铁钽矿结构（I2/a空间群，Z = 2）。X射线衍射和拉曼光谱测量均表明，在临界浓度（xc）下，x浓度的增加使结构从单斜铁钽矿转变为四方白钨矿（I41/a空间群，Z = 4）。单斜相和四方相在0.06≤x < xc的组成范围内共存。分别采用哈基 - 科尔曼和反射腔技术测量了这些稳定相介电常数和品质因数。通过使用连接到可编程热板的因瓦腔测量谐振频率的温度系数。高密度样品具有良好的微波介电性能。
