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K R Poeppelmeier

Publications and source records attributed to K R Poeppelmeier.

7 recordsLinked to original sources
PubMed2001

Polar compounds constructed with the [Cr2O7]2- anion.

Crystalline KTiOPO4 (KTP), an inorganic nonlinear optical material with a waveguide figure-of-merit that is twice that of other mixed-metal oxides, contains helical chains of TiO(4/2)O(2/2) octahedra in which a long, short Ti-O bond motif results in a net c-directed polarization. The alternating long and short Ti-O bonds that occur along these chains are the major contributors to the large nonlinear optic and electrooptic coefficients. Analogous chains have been constructed using dichromate [Cr2O7]2- anions and [M(py)4]2+ (M = Cu, Zn) cations; these new transition metal oxides crystallize in the same space group as KTP. Crystal data for Cu(py)4Cr2O7: orthorhombic, space group Pna2(1) (No. 33), with a = 15.941(7) A, b = 16.324(3) A, c = 8.857(2) A, and Z = 4; for Zn(py)4Cr2O7, orthorhombic, space group Pna2(1) (No. 33), with a = 16.503(1) A, b = 16.005(1) A, c = 8.8130(5) A, and Z = 4; for Cd(py)4Cr2O7, monoclinic, space group C2/c (No. 15), with a = 14.8034(9) A, b = 11.1847(7) A, c = 15.788(1) A, beta = 110.023(1) degrees, and Z = 4.

Journal Article↗
PubMed2000

Cation disorder in Ga1212.

Substitution of calcium for strontium in LnSr2-xCaxCu2GaO7 (Ln = La, Pr, Nd, Gd, Ho, Er, Tm, and Yb) materials at ambient pressure and 975 degrees C results in complete substitution of calcium for strontium in the lanthanum and praseodymium systems and partial substitution in the other lanthanide systems. The calcium saturation level depends on the size of the Ln cation, and in all cases, a decrease in the lattice parameters with calcium concentration was observed until a common, lower bound, average A-cation size is reached. Site occupancies from X-ray and neutron diffraction experiments for LnSr2-xCaxCu2GaO7 (x = 0 and x = 2) confirm that the A-cations distribute between the two blocking-layer sites and the active-layer site based on size. A quantitative link between cation distribution and relative site-specific cation enthalpy for calcium, strontium, and lanthanum within the gallate structure is derived. The cation distribution in other similar materials can potentially be modeled.

Journal Article↗
PubMed2000

Site-specific vanadates Co4Fe3.33(VO4)6 and Mn3Fe4(VO4)6.

Single crystals of Co4Fe3.33(VO4)6 and Mn3Fe4(VO4)6 were grown from equivalent CoO/Fe2O3/V2O5 and MnO/Fe2O3/V2O5 melts, respectively. The former crystallizes in the orthorhombic space group Pnma with parameters a = 4.965(1) A, b = 10.211(1) A, c = 17.188(3) A, and Z = 2 and is a homeotype of such catalysts as Mg2.5VMoO8. The latter crystallizes in the triclinic space group P1 with parameters a = 6.703(2) A, b = 8.137(1) A, c = 9.801(2) A, alpha = 105.56(1) degrees, beta = 105.58(2) degrees, gamma = 102.35(1) degrees, and Z = 1 and is a homeotype of beta-Cu3Fe4(VO4)6, the low-pressure form of alpha-Cu3Fe4(VO4)6. The cobalt analogue deviates in stoichiometry from the reactant melt to form the more dense alpha-Cu3Fe4(VO4)6 structure type comprised of partially occupied face-sharing octahedral and trigonal prismatic coordination sites.

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