New Noncentrosymmetric Vanadates Sr₉R(VO₄)₇ (R = Tm, Yb, and Lu): Synthesis, Structure Analysis, and Characterization

New vanadates Sr9R(VO4)7 (R = Tm, Yb, and Lu) were synthesized using a standard solid-state method at 1373 K and found to be isotypic with Ca3(VO4)2 at room temperature (RT). Their structure parameters were refined using the Rietveld method from synchrotron X-ray diffraction (XRD) data measured at RT (space group R3c and Z = 6). Sr9R(VO4)7 (R = Y and La−Er) do not form a phase isotypic with Ca3(VO4)2. Sr9R(VO4)7 (R = Tm, Yb, and Lu) were characterized through the magnetic susceptibility (2−400 K), the specific heat (0.45−31 K), thermal analysis (300−1573 K), and high-temperature XRD, second-harmonic generation, and dielectric measurements. The temperature dependence of the dielectric constant and tangent loss suggested that they exhibit a reversible ferroelectric−paraelectric phase transition of the first order near 950−960 K. The high-temperature phases have space group R3̄m and Z = 3. Thermal analysis revealed the presence of an intermediate phase between the R3c and R3̄m phases in a very narrow temperature range. Magnetic susceptibilities of Sr9Tm(VO4)7 and Sr9Yb(VO4)7 are typical of Tm3+ and Yb3+ ions affected by an octahedral crystal field. The effective magnetic moments were 7.39 μB for Tm3+ and 4.59 μB for Yb3+