Elucidating elusive quaternary selenide EuCeCuSe3:Synthesis,crystal structure,properties and theoretical studies

We report on the novel heterometallic quaternary selenide EuCeCuSe3,the fabrication of which has been a challenge until this work.The structure of the reported selenide was elucidated from the powder X-ray diffraction data,which revealed the formation of EuCeCuSe3with excellent yield(96.7%)accompanied with a minor fraction of CeSe2(3.3%),and was best solved in orthorhombic space group Pnma with the BaLaCuS3structural type.Thus,the crystal structure of the title compound completes the row of the heterometallic quaternary selenides EuRECuSe3(RE=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu,Y),of which the cerium-based derivative exclusively belongs to the BaLaCuS3structural type.The distortion of the CuSe4polyhedron was compared for the whole series of EuRECuSe3compounds using theτ4-descriptor for four coordinated ions,which revealed the highest degree of distortion for the Ce3+-containing selenide,followed by the La3+-based derivative.Furthermore,the crystallographic and geometrical parameters of the reported selenide were discussed in comparison to the Ce3+-based sulfides SrCeCuS3and EuCeCuS3.Ab initio calculations of the crystal structure,a phonon spectrum and elastic constants for the crystal of EuCeCuSe3were also performed.The types and wavenumbers of fundame ntal modes were determined and the involvement of ions participating in the phonon modes was assessed.The experimental IR spectrum of the reported selenide was interpreted and found to be in agreement with the calculated spectrum.The experimental direct band gap of EuCeCuSe3was measured to be 1.36 eV that is consistent with the concept of its origin due to interband transitions between orbitals emerging mainly from 4f(valence band)and 5d(conduction band)levels of the Eu2+cation.The dependence of the Young’s modulus on the direction demonstrates the anisotropy of the elastic properties,while the Vickers hardness for EuCeCuSe3was calculated to be 5.2 GPa.Finally,the title compound is paramagnetic above 4 K.

Negative thermal expansion in one-dimension of a new double sulfate AgHo(SO_(4))_(2)with isolated SO_4 tetrahedra

A double holmium-silver sulfate was obtained for the first time.The temperature intervals for the formation and stability of the compound were determined by differential scanning calorimetry.The crystal structure of AgHo(SO_(4))_(2)was determined by Rietveld method.The X-ray diffraction(XRD)analysis showed that the compound crystallizes in the monoclinic syngony,space group P2_1/m,with the unit cell parameters a=4.71751(4)A,b=6.84940(6)A and c=9.89528(9)A,β=95.1466(4)·,V=318.448(5)A^(3),Z=2,R_B=1.55%,T=303 K.Two types of sulfate tetrahedra were found in the structure,which significantly affected the spectral properties in the infrared range.In the temperature range of 143-703 K,a negative thermal expansion along the b direction accompanied by a positive thermal expansion along the a and c directions was observed.It was established that negative thermal expansion is the result of the deformation of sulfate tetrahedra,which is affected by the movement of holmium and silver atoms.The excitation in the blue spectral range(457.9 nm)produces a luminescence in light blue(489 nm),green(545 nm)and red(654 nm)spectral ranges,and the latter two were of comparable intensity that is favorable for WLED sources.The observed luminescent band distribution is ascribed to the specific crystal field at Ho^(3+)ion sites rather than a variation of radiationless probability.