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+.
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