The High‐Pressure Polymorph of Ca4Te5O14 and the Mixed‐Valent Compound Ca13TeVI2/3TeIV3.75O15(BO3)4(OH)3

A high‐pressure/high‐temperature experiment based on the multi‐anvil technique has yielded a high‐pressure (HP) modification of Ca 4 Te 5 O 14 ( I ) from the starting materials Ca(OH) 2 and TeO 2 . Reaction with the boron nitride crucible used in the high‐pressure set‐up led also to the formation of the mixed‐valent Te IV,VI borate Ca 13 Te VI 2/3 Te IV 3.75 O 15 (BO 3 ) 4 (OH) 3 ( II ) as a minor phase. The crystal structures of both compounds were determined by single‐crystal X‐ray diffraction [ I : P 2 1 / c , Z = 4, a = 11.0272(1), b = 12.0588(1), c = 10.1038(1) Å, β = 91.849(1)°, R 1( F 2 ) = 0.0317, wR 2( F 2 ) = 0.0459; II : P 6 3 / m, Z = 2, a = 9.0890(13), c = 18.731(4) Å, R 1( F 2 ) = 0.0401, wR 2( F 2 ) = 0.0867]. The principal building units of HP‐Ca 4 Te 5 O 14 , namely [CaO x ] ( x = 7, 8) and Te IV O 3 and Te IV O 4 polyhedra, are similar to those of the ambient modification, but the structural set‐up is markedly different in the two structures with an increase in density of about 6.6 %. Ca 13 Te VI 2/3 Te IV 3.75 O 15 (BO 3 ) 4 (OH) 3 is the first compound known so far to contain both oxotellurate and oxoborate units. The most remarkable feature of its disordered crystal structure is the centrosymmetric Te 3 O 6 unit consisting of a Te VI O 6 octahedron sharing opposite faces with two trigonal Te IV O 3 pyramids. The face‐sharing of Te VI and Te IV oxopolyhedra is unprecedented in the crystal chemistry of oxotellurates. In both structures I and II , the lone‐electron pairs of the Te IV atoms are stereochemically active.