Ambient-Pressure Synthesis of Two New Vanadium-Based Calcium Ferrite-Type Compounds: NaV_1.25Ti_0.75O₄ and NaVSnO₄

Two new CaFe<inf>2</inf>O<inf>4</inf>-type compounds NaV<inf>1.25</inf>Ti<inf>0.75</inf>O<inf>4</inf> (1) and NaVSnO<inf>4</inf> (2) have been prepared at ambient pressure and temperatures < 800 °C. This contrasts with the parent material NaV<inf>2</inf>O<inf>4</inf> which is synthesized at 6 GPa and 1300 °C. The lattice parameters are a = 9.1500(4) Å, b = 2.9399(3) Å, and c = 10.6568(5) Å for 1 and a = 9.3083(6) Å, b = 3.0708(2) Å, and c = 10.9194(5) Å for 2 (space group Pnma). Structure refinement against neutron powder diffraction data reveals that V/Ti and V/Sn are disordered over two octahedral sites. Both materials are characterized by a magnetic transition near 150 K below which the Curie moment is reduced from a value consistent with V³⁺ [0.75 emu mol<inf>V</inf>^-1 K^-1 for 1 and 0.58 emu mol<inf>V</inf>^-1 K^-1 for 2] to 0.23 emu mol<inf>V</inf>^-1 K^-1 for 1 and 0.30 emu mol<inf>V</inf>^-1 K^-1 for 2, signaling a 70-50% reduction in the paramagnetic moment. The Weiss temperature (θ) is reduced from -285 (1) and -138 K (2) to values close to 0 K, suggesting that the remaining spins are dilute and weakly interacting. Heat capacity measurements reveal a gradual loss of magnetic entropy between 2 and 150 K, consistent with short-range bulk magnetic ordering. In addition, heat capacity and magnetic susceptibility measurements reveal a number of weak magnetic transitions below 6 K involving both antiferromagnetic and ferromagnetic components.