K8Nb7P7O39: a new type of complicated incommensurately modulated structure and photoluminescence properties of Eu3+-doped solid solutions

Abstract

A new type of potassium niobium phosphate, K₈Nb₇P₇O₃₉, has been prepared and its structure has been determined by single crystal X-ray diffraction analysis. The results show that it crystallizes in a novel (3 + 1)-dimensional incommensurately modulated structure with a triclinic superspace group P(αβγ)0 and modulation vector q = −0.35281a* + 0.17630b* + 0.25807c*. The structure of K₈Nb₇P₇O₃₉ contains [K_3.66]_∞, [K_4.34]_∞ and [Nb₇P₇O₃₉]_∞ layers alternately stacked along the c-axis. In this structure, seven not fully occupied K atoms (six of them come from the [K_3.66]_∞ layer and the remaining one comes from the [K_4.34]_∞ layer) are thought to be the dominant factors in driving the structural modulation. The strong occupational modulation in combination with positional modulation of these seven K atoms can be described by crenel-type modulation functions. Furthermore, elemental composition, UV-Vis absorption spectrum, SEM morphology and photoluminescence properties induced by Eu³⁺-doping have been studied for K₈Nb₇P₇O₃₉. The excitation spectrum covers a wide range from 362 to 465 nm and it matches well with the emission of near-UV InGaN-based LED chips and bluish-green LED chips, indicating that K₈Nb₇P₇O₃₉:Eu³⁺ has potential as a red-emitting phosphor for LED applications.