Structural investigation of R2Mo 4O 15 ( R=La, Nd, Sm), and polymorphs of the R2Mo 4O 15 ( R=rare earth) family

Abstract

Pyrolysis of rare earth ( R) polyoxomolybdate, [ R 2(H 2O) 12Mo 8O 27]· xH 2O ( R=La, Nd and Sm), at 750°C for 2–8 h results in crystallization of R 2Mo 4O 15 compounds. β-La 2Mo 4O 15 crystallizes together with an α-form in monoclinic P2 1/ a (No. 14), a=13.8893(5), b=13.0757(4), c=20.0927(8) Å, β=95.199(2)°, V=3634.1(2) Å 3, Z=12, R 1( I>2 σ( I))=0.048, R w (all data)=0.116. The structure is built up with {LaO n } ( n=9, 10) and {MoO n′ } ( n′=4–6) polyhedral units. The {LaO n } units are polymerized into a linear {La 6O 39} ∞ chain, while the {MoO n } are connected together to form {Mo 4O 15} and {Mo 7O 26} groups. The structure can be related to the α-form by partial rearrangement of O atoms and small shifts of La and Mo atoms. The R 2Mo 4O 15 ( R=Nd and Sm) compounds are isomorphous with the previously reported R=Eu and Gd analogs, crystallizing in triclinic, P 1 ̄ (No. 2), a=9.4989(5) and 9.4076(7), b=11.0088(7) and 10.9583(8), c=11.5665(6) and 11.5234(8) Å, α=104.141(3) and 104.225(3), β=109.838(3) and 109.603(3), γ=108.912(3) and 108.999(3)°, V=987.3(1) and 970.5(1) Å 3, Z=3, R 1( I>2 σ( I))=0.028 and 0.030, R w (all data)= 0.079 and 0.094, respectively. The crystal structure is composed of { RO 8} and {MoO n′ } ( n′=4–6) polyhedral units. The molybdate units are condensed to give a corrugated {Mo 4O 17} ∞ chain. The square-antiprismatic { RO 8} units share their trigonal and square faces, forming { R 2O 13} and { R 2O 12} groups, respectively. A very short R⋯R distance (3.557(6) Å for R=Nd; 3.4956(6) Å for R=Sm) is achieved in the latter unusual { R 2O 12} group. A common cationic arrangement was found in all the structures in the R 2Mo 4O 15 family: a R–R pair with the shortest separation and surrounding 12 Mo atoms. The symmetry of the cationic arrangement was reduced with an increase of atomic number of R, viz. La>Ce, Pr>Nd–Gd≈Tb, Ho.