Two new microcrystalline (oxo)nitridosilicates with complex crystal structures determined by combination of
TEM
and synchrotron micro diffraction
Authors: Neudert Lukas , Peter Schultz , Durach Dajana , Oeckler Oliver , Schnick Wolfgang 
Published: December 2016

Nitridosilicates and oxonitridosilicates doped with rare earth elements are well known as luminescent materials, e.g. in phosphor‐converted LEDs.
[1]
The synthesis of such complex multinary materials often leads to inhomogeneous microcrystalline samples that contain unknown compounds. With respect to structure‐property relationships, precise structure determination is essential to predict and understand luminescence properties.

Two novel La,Ba‐(oxo‐)nitridosilicates were found in a microcrystalline sample. They are coherently intergrown which renders structure determination challenging. The two phases were identifies by selected‐area electron diffraction (SAED) and X‐ray spectroscopy (EDX). Suitable (not intergrown) single crystals of each phase were located in TEM images. Lattice parameters and the chemical composition were determined. Both phases show hexagonal metrics and exhibit the same
c
lattice parameter; however, they differ in the
a
lattice parameter and the chemical composition (Fig. 1). Single‐crystal data of these crystallites on TEM grids were collected using sub‐micron synchrotron beams (ID11, ESRF, Grenoble).
[2]
Corresponding structure solution yielded two structure models. These were confirmed by HRTEM along characteristic directions, including corresponding simulations, as well as Z‐contrast imaging (STEM‐HAADF, Fig. 1). The latter shows heavy atom sites (La, Ba). For both compounds, the plane groups of HRTEM images (along [001]:
p
3) and the symmetry of SAED patterns pointed to the Laue class 6/
m
. The distribution of elements with lacking scattering contrast such as O/N and Ba/La on mixed occupied sites were analyzed by bond‐valence sum calculations.

The oxonitridosilicate Ba
24
La
54
Si
129
N
240
O
3
(
a
= 17.49 Å,
c
= 22.70 Å) shows an interrupted, centrosymmetric 3D framework of vertex‐ and edge‐sharing Si(O,N)
4
tetrahedra. The crystal structure can be described assuming two slab‐like building blocks (Fig. 2A). Slab a consists of two layers of
dreier
rings that are interconnected via pairs of Si(O,N)
4
tetrahedra. Slab c forms asymmetric
dreier
, vierer,
fünfer
,
sechser
and
siebner
rings of Si(O,N)
4
tetrahedra and contains the edge‐sharing Si(O,N)
4
tetrahedra. These two blocks are interconnected via an additional layer (part b) that formally consists of isolated groups of four Si(O,N)
4
tetrahedra.

The second phase is a nitridosilicate (Ba
29
La
80
Si
173
N
330
with
a
= 20.19 Å,
c
= 22.68 Å). Comparable to Ba
12
La
14
Si
43
N
76
O
5
, the crystal structure consists of an interrupted 3D network of SiN
4
tetrahedra, which can also be divided in different parts. The main difference is part c, especially concerning the arrangement and linkage of the edge sharing SiN
4
tetrahedra. The difference is caused by the pairs of Si(O,N)
4
tetrahedra parallel to (Fig. 2B, white tetrahedra) in Ba
24
La
54
Si
129
N
240
O
3
which are replaced by three SiN
4
tetrahedra linked over one Si atom (Fig. 2C) in Ba
29
La
80
Si
173
N
330
. The pronounced tendency towards intergrowth of both phases is due to the fact that part a and b are very similar in both structures.