Abishek K. Iyer

Ba5CdGa6Se15, a congruently-melting infrared nonlinear optical material with strong SHG response

The quaternary selenide Ba5CdGa6Se15 was prepared by reaction of BaSe and CdGa2Se4 at 1273 K and forms as light yellow crystals. It adopts a noncentrosymmetric structure (orthorhombic, space group Ama2, Z = 4, a = 24.2458(8) A, b = 19.1582(7) A, c = 6.6028(2) A) featuring a three-dimensional anionic framework built up of corner-sharing tetrahedra centred by Cd and Ga atoms, with the intervening voids filled by Ba cations. The Cd atoms are partially disordered with Ga atoms within two out of five possible sites; the site preference can be understood in terms of optimizing bond valence sums. Band structure calculations predict that Ba5CdGa6Se15 should be a semiconductor in which substitution of Cd for Ga atoms reduces the electron count in a hypothetical all-Ga compound “Ba5Ga7Se15” so that the Fermi level falls within a band gap. Ba5CdGa6Se15 is a potential new infrared nonlinear optical material with several attractive properties. First, the large band gap of 2.60(2) eV, as determined from the UV-vis-NIR optical absorption spectrum, implies a high laser damage threshold. Second, strong second harmonic generation signals were observed from nonlinear optical measurements on powder samples, with intensities comparable to that of the benchmark material AgGaS2; type-I phase-matching behaviour was confirmed using 2090 nm as the fundamental laser wavelength. Third, this compound melts congruently at a relative low temperature of 866 °C, which could facilitate growth of large crystals eventually required for applications.

Not Just Par for the Course: 73 Quaternary Germanides RE4M2XGe4 (RE = La–Nd, Sm, Gd–Tm, Lu; M = Mn–Ni; X = Ag, Cd) and the Search for Intermetallics with Low Thermal Conductivity

A total of 73 new quaternary rare-earth germanides RE4 M2 XGe4 ( RE = rare-earth metal; M = Mn-Ni; X = Ag, Cd) were prepared through reactions of the elements. The solid solution Nd4Mn2Cd(Ge1- ySi y)4 was also prepared under the same conditions and found to be complete over the entire range. All of these compounds adopt the monoclinic Ho4Ni2InGe4-type structure (space group C2/ m, a = 14.2-16.7 Å, b = 4.0-4.6 Å, c = 6.8-7.5 Å, β = 106-109°), as revealed by powder X-ray diffraction analysis and single-crystal X-ray diffraction analysis on selected members. The structure determination of Nd4(Mn0.78(1)Ag0.22(1))2Ag0.83(1)Ge4 disclosed disorder of Mn and Ag atoms within the tetrahedral site and Ag deficiencies within the square planar site. Within the solid solution Nd4Mn2Cd(Ge1- ySi y)4, the end-members and two intermediate members were structurally characterized; as the Si content increases, the Cd sites become less deficient and the individual [Mn2 Tt2] layers contract but become further apart from each other. Electronic band structure calculations confirm that the Ag-Ge or Cd-Ge bonds are the weakest in the structure and thus prone to distortion. Thermal property measurements confirm expectations from machine-learning predictions that these quaternary germanides should exhibit low thermal conductivity, which was found to be <10 W m-1 K-1 for Nd4Mn2AgGe4.