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Structure D 2000Synthesis and Investigation of Tin(II) Pyrophosphate Sn 2 P 2 O 7 . -The title compound is prepared by heating a stoichiometric mixture of SnO and NH4H2PO4 at 820 K for 24 h and characterized by single crystal and powder XRD, and 119 Sn Moessbauer spectroscopy. The low-temperature β-modification crystallizes in the triclinic space group P1 with Z = 4. The structure contains [P2O7] 4groups oriented in mutually perpendicular directions with Sn atoms situated in large structural interstices. The title compound undergoes a structural transition at 623 K. The high-temperature α-form crystallizes in the monoclinic space group P21/n with Z = 2. -(CHERNAYA, V. V.; MITIAEV, A. S.; CHIZHOV, P. S.; DIKAREV, E. V.; SHPANCHENKO*, R. V.; ANTIPOV, E. V.; KOROLENKO, M. V.; FABRITCHNYI, P. B.; Chem. Mater. 17 (2005) 2, 284-290; Dep. Chem., Moscow State Univ., Moscow 119889, Russia; Eng.) -W. Pewestorf

The structural investigation of Sn 2 P 2 O 7 was carried out for the first time by means of single crystal and powder X-ray diffraction. The crystal structure of β-Sn 2 P 2 O 7 pyrophosphate was solved using single-crystal X-ray data at room temperature and at 93 K. At room temperature the structure (triclinic, P1 h, a ) 5.2776(5) Å, b ) 11.5413(12) Å, c ) 11.6360(12) Å, R ) 102.911(8)°, β ) 99.303(8)°, γ ) 98.899(8)°, V ) 668.2(3) Å 3 , Z ) 4) contains the [P 2 O 7 ] 4pyrophosphate groups oriented in mutually perpendicular directions with tin atoms situated in large structural interstices. This structure remains intact at low temperatures. The Sn 2 P 2 O 7 undergoes a reversible structural transition at 623 K found by thermal analysis which is accompanied by increase of the unit cell symmetry to monoclinic. The crystal structure of the high temperature R-Sn 2 P 2 O 7 form was solved using high-temperature powder diffraction data collected at 773 K. The structural motif of the high temperature form (monoclinic, P2 1 /n, a ) 7.1765(4) Å, b ) 9.2874(6) Å, c ) 5.2968(4) Å, β ) 106.034(3)°, V ) 339.30(5) Å 3 , Z ) 2) is closely related to the room temperature modification. The stereochemically active lone pairs of the tin atoms were localized with the ELF calculations for both polymorph modifications. In addition, the electronic state of tin was characterized using 119 Sn Mo ¨ssbauer spectroscopy.