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 Ca-3(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 Ca-3(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 R (3) over barm and Z = 3. Thermal analysis revealed the presence of an intermediate phase between the R3c and R (3) over barm 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 mu(B) for Tm3+ and 4.59 mu(B) for Yb3+.