La₈₊<i>_x</i>Ti₈₊<i>_y</i>S₂₄O₄ Compounds Where <i>x</i> + <i>y</i> ≤ 2:  A Series of Phases with Mixed-Valent Titanium

Abstract

Three new titanium oxysulfides, La 8.75 Ti 9.25 S 24 O 4, La 8.50 Ti 9.50 S 24 O 4, and La 8.10 Ti 8.05 S 24 O 4, were prepared by high-temperature methods (1025−1050 °C; ∼80−150 h) from La 2 S 3, Ti, S, and either La 2 O 3 or TiO 2 as oxygen sources. LaCl 3 (∼10−30 wt %) was added as a flux. The compounds were characterized by single-crystal X-ray diffraction, neutron activation analysis, and EDX analysis. All three compounds contain common [(Ti 4 S 2 O 4 )(TiS 6 ) 4/2 ] (12+ δ ) - oxysulfide layers and disordered/variable composition titanium sulfide layers that alternately stack along the c -axis of the unit cell. The titanium sulfide layers contain extensive La/Ti solid solution formation and are characterized by 5-fold disorders at the M(2) octahedral sites and 2-fold disorders at the M(3) sites. These compounds are members of a series of compounds with the general formula La 8+ x Ti 8+ y S 24 O 4, where (x + y) ≤ 2 and the oxidation states of Ti can vary from +3 to +4. Crystal data (153 K) of La 8.75 Ti 9.25 S 24 O 4: tetragonal space group P 4/ mmm, a = 10.4205(8) Å, c = 8.3848(6) Å, V = 910.5(1) Å 3, Z = 1, R(F) = 3.88% and wR(F 2 ) = 7.22%. For La 8.50 Ti 9.50 S 24 O 4, tetragonal space group P 4/ mmm, a = 10.4101(3) Å, c = 8.396(1) Å, V = 909.9(1) Å 3, Z = 1, R(F) = 3.54% and wR(F 2 ) = 8.16%. For La 8.10 Ti 8.05 S 24 O 4, tetragonal space group P 4/ mmm, a = 10.4212(3) Å, c = 8.233(2) Å, V = 894.1(2) Å 3, Z = 1, R(F) = 4.25% and wR(F 2 ) = 11.12%.