La11Si11P21Te5 and La20.667Si18P46Te11.333 : two novel 3D‐polyanionic lanthanum compounds following the Zintl‐Klemm concept

Abstract

During attempts to synthesize La3PTe3, after participation of the SiO2-container material, two so far unknown compounds could be isolated, which both follow the Zintl-Klemm concept. The novel quaternary lanthanum tellurides La11Si11P21Te5 and La20.667Si18P46Te11.333 crystallize hexagonally in the non-centrosymmetric space group Pmathematical equation c with the unit-cell parameters a=1204.97(6)-pm and c=1802.38(9)-pm (c/a=1.496) or a=1205.78(6)-pm and c=1802.16(9)-pm (c/a=1.495), respectively. Their very similar crystal structures can be divided into two interpenetrating three-dimensional sublattices, mathematical equation {[La11Te5]23+} (d(La3+-Te2-)=314-335-pm plus 352-pm) and mathematical equation {[Si11P21]23-} (d(Si-P)=221-225-pm, d(Si-Si)=230-231-pm, d(P-P)=220-221-pm) for La11Si11P21Te5 (Z=2), displaying unique structural features as compared to related compounds. The linkage of these two sublattices occurs through attractive La3+-Si- and La3+-P- interactions (d(La-Si)=333-pm, d(La-P)=298-318-pm). For La20.667Si18P46Te11.333 (Z=1), 4/3 out of 22 La3+cations in the doubled formula La22Si22P42Te10 have to be omitted, while the four Si- anions need to be replaced with phosphorus atoms. The La3+ vacancies are filled up with tellurium, partly forming [Te2]2- dumbbells (d(Te-Te)=274-pm). La11Si11P21Te5 can be obtained on target by reacting the elements (La, Si, P and Te) in their respective amounts with an excess of potassium chloride as fluxing agent in evacuated and torch-sealed glassy silica ampoules for four days at 900 °C as black pillars or thick plates with hexagonal cross-section.