Cation ordering and ferroelectric-type phase transitions in new Sr-based whitlockite-related Sr9–xBaxR(VO4)7 (R = In, Yb) vanadates

New solid solutions of Sr 9- x Ba x In(VO 4 ) 7 (SBIV) and Sr 9- x Ba x Yb(VO 4 ) 7 (SBYV) with whitlockite-type structure were obtained by solid-phase synthesis within the ranges of 0 ≤ x ≤ 1.75 and 0 ≤ x ≤ 2, respectively. Incorporation of Ba 2+ cations reduces phase transition temperatures from 959 K to 826 K for SBIV, and from 950 K to 800 K for SBYV. All synthesized samples showed nonlinear second harmonic generation (SHG) efficiency 10 times more than that of the quartz standard. SHG response remained largely constant as x increased. Sr 2+ cation conductivity is suggested for both SBIV and SBYV. The incorporation of barium cations hinders the migration paths for Sr 2+ cation. Barium atoms are located predominantly in the M 3 position, which complicates the migration paths for strontium. The key result is creating a model for future finding of Sr-based ion conducting materials.