New Solids in As-O-Mo, As(P)-O-Mo(W) and As(P)-O-Nb(W) Systems That Exhibit Nonlinear Optical Properties

Interactions between well-mixed fine powders of As2O3, P2O5, MoO3, WO3 and Nb2O5 at different stoichiometry in quartz ampoules under vacuum at ~1000 °C in the presence of metallic molybdenum (or niobium), over several weeks, led to shiny dichroic crystalline materials being formed in cooler parts of the reaction vessel. An addition of small quantities of metals-Mo or Nb-was made with the aim of partially reducing their highly oxidized Mo(VI), W(VI) or Nb(V) species to corresponding Mo(V), W(V) and Nb(IV) centers, in order to form mixed valence solids. Sublimed crystals of four new compounds were investigated using a variety of techniques, with prime emphasis on the X-ray analysis, followed by spectroscopy (diffusion reflectance, IR, Raman and EPR), second harmonic generation (SHG), thermal analysis under N2 and air atmosphere, and single crystals electrical conductivity studies. The results evidenced the formation of new complex solids of previously unknown compositions and structures. Three out of four compounds crystallized in non-centrosymmetric space groups and represent layered 2D polymeric puckered structures that being stacked on each other form 3D lattices. All new solids exhibit strong second-harmonic-generation (SHG effect; based on YAG 1064 nm tests with detection of 532 nm photons), and a rare photosalient effect when crystals physically move in the laser beam. Single crystals’ electrical conductivity of the four new synthesized compounds was measured, and the results showed their semiconductor behavior. Values of band gaps of these new solids were determined using diffusion reflectance spectroscopy in the visible region. Aspects of new solids’ practical usefulness are discussed.


SM 1 Figure 1
Principal outcome of the synthesis of new mixed metal, mixed oxides solids carried out in a quartz ampoule. Powder residue was found inhomogeneous and was discarded. Crystals used for studies.

SM 2 Figures 2,3
Experimental setup of the second harmonic generation (SHG) measurements: top -block scheme; bottom -actual laboratory settings showing the videomicroscope for optical alignment of single crystals in the beam, with a red arrow.

SM 3 Figures 4,5
Plotted data of observed SHG effect in rotating of sample 1 with its polarization measurements (A). Time evolution of the SHG signal at 532 nm from single crystals of sample 4 (B); the SHG signal becomes very small within 2-3 minutes where decrease of its intensity is indicated with arrows.

A B SM 4 Chart 1
We observed a photosalient effect in all studied single crystals of new non-centrosymmetric solids. It is a new phenomenon some details of which were first aired and classified in Angewandte Chemie International Edition (then in Photonics.com, June 2014), and in brief presented in diagram below.
It was first observed in crystals of Werner-type complex [Co(NH 3 ) 3 (NO 2 )]Cl(NO 3 ), where single crystals underwent sudden jumps over distances 10 2 to 10 5 times their own size.

SM 5 Figure 6
The explanation for observed mode 4 photosalient effect in studied new mixed metal oxides. Compound 4 is used as an example.
Unit cell constants b and c in this orthorhombic cell are not changing much, while the shortest dimension a does change considerably. This is the distance between layers, or puckered plates, in the structure, which turned out to be sensitive to temperature. Table 1 Key EPR active isotopes of the most common transition metals used in materials chemistry research and their EPR appearances. Used in this work elements are indicated with arrows.

SM 7 Figure 7
Low temperature EPR spectrum of crystalline compound 3 showing overlapping signals. Shown coupled pentavalent molybdenum and tungsten centers.

SM 8 Figure 8
Two different metal sites (marked as 1 and 2) in the structure of compound 4 (a), its EPR spectrum at 80K, and fitting (b), showing unresolved isotopic hyperfine coupling.

SM 9 Figure 9
Low temperature EPR spectra of crystalline compound 4 before and after recording its thermogramm under pure N 2 atmosphere. Blue spectrum contains unresolved hyperfine coupling of one unpaired electron of Mo(V) species with 95 Mo isotope nucleus. Table 2 Table of standard redox potentials for elements involved in synthesis of mixed metal, mixed valence oxides.

SM 11 Figures 10, 11
Micro Full line shape analysis for solid state reflectance spectrum of compound 2 showing deconvoluted peaks and overall fitting statistics. The minimum number of five Gaussian-type peaks is necessary to fit well experimental curve (black). Red trace is the sum of theoretical fit, while blue peaks represent individual components of spectral envelope.

SM 16 Figure 19
Prospective view of the unit cell content using polyhedrons' representation of the crystal structure of compound 1.

SM 17 Figure 20
Two orthogonal views of the GROW fragment in the structure of 1. Green arrows show some corner-sharing octahedrons in the structure, while yellow arrows show edge-sharing places for junction.

SM 18 Figure 21
Metallocycles in the structure of 1: analysis of planarity of the core. A -top view, B -side view, C -dihedral angle in eight-membered cycle in the middle of the structure where 2-fold rotation axis is.

A B C SM 19 Figure 22
Metallocycles in the structure of 1: analysis of planarity of three six-membered cycles in the core.

SM 20 Figure 23
Side (A) and top (B) views of pnictogens centers in the structure of 1 showing highly distorted trigonal bipyramidal geometry.

SM 21 Figure 24
Geometry of a pentagonal star shaped formation in the structure of 1 angles between pnictogen atoms As/P and oxygen atoms -side view A and top view B, and bond lengths C. Shared sites between transition metals and pnictogens are indicated with green arrows.

A B SM 22 Figure 25
Geometry of a pentagonal star shaped formation in the structure of 1 angles between pnictogen atoms showing bond lengths.

SM 23 Figure 26
Analysis of structure of 1: values of principal bonds and angles at individual metal centers with SOF = 1, Nb1 and Nb2

SM 24 Figure 27
Analysis of structure of 1: values of principal bonds and angles at individual metal centers with SOF = 1, Nb3, Nb7.

SM 25 Figure 28
Literature data analysis of polyhedrons distortions: selected bond lengths and valence angle in the structure of N 2 O 5 (monoclinic, P1) for comparison with the structure of 1.

SM 26 Figure 29
Literature data analysis of polyhedrons distortions: selected bond lengths and valence angle in the structure of WO 3 (monoclinic, P2 1 /c) for comparison with same parameters in the structure of 1.  [3]) (see the Reference Section at the end).

SM 27 Chart 2
Organization of 2D crystal lattice in the structure of compound 1: thick plates are stacked on a top of other with different averaged E-O distances (E = As, P, Mo, W) in between forming layered structure. Each plate represents puckered 2D net.
The averaged distance inside the ASU net based on 17 data (blue plate), with upper pink plate based on 11 data, and lover brown plate based on 8 data as generated by the Mercury Crystallographic Viewing software.

SM 28 Figure 30
The videomicroscope scaling of the needle-type specimen of compound 2 used for crystallographic studies. Indexing crystal faces led to accurate dimensions determinations that were used in SADABS procedure for absorption correction.

SM 29 Figure 31
Polyhedral representation of the structure of 2: view along b-direction. Oxygen atoms of occlusion are indicated in red.

SM 30 Figure 32
Two orthogonal views of the GROW fragment in the structure of 2. Corner-sharing octahedrons are clearly seen in the structure, while yellow arrows show corner-sharing tetrahedrons of PO 4fragment for junction.

SM 31 Figure 33
Analysis of structure of 2: values of principal bonds and angles at individual metal centers.

SM 32 Figure 34
Analysis of structure of 2: values of principal bonds and angles at individual metal centers (continued).

SM 33 Figure 35
The polyhedrons' representation of the crystal structure of compound 3.

SM 34 Figure 36
Two orthogonal views of the GROW fragment in the structure of 3. Corner-sharing octahedrons are clearly seen in the structure, while yellow arrows show edge-sharing As-containing tetrahedrons with MoO6 octahedrons.

SM 35 Figure 37
Analysis of structure of 3: values of principal bonds and angles at individual metal centers.

SM 36 Figure 38
Analysis of structure of 3: values of principal bonds and angles at individual metal centers (continued).

SM 37 Figure 39
Analysis of structure of 3: values of principal bonds and angles at individual metal centers (continued).

SM 38 Figure 40
Literature data analysis of polyhedrons distortions: selected bond lengths and valence angle in the structure of MoO 3 (monoclinic, P2 1 /n) for comparison with same parameters in structures of 2 -4.

SM 40 Figure 42,43
Core of the structure of 3: polymeric 2D net of Mo/W and O atoms: Top view (A) and side view (B).

43:
Details of geometry of the As1 atom. Place for the lone pair is inferred as well.

A B SM 42 Figure 45
Two orthogonal views of the GROW fragment in the structure of 4. Only corner-sharing octahedrons are clearly seen in the structure. Arrows point out on location of As-atoms.

SM 44 Figure 47
Some experimental details for single crystals electrical conductivity measurements: Glass cover slip with single crystal of compound 2 and graphite glue/paste with embedded golden wires.
Shown original crystal (inset) and glued specimen (below)

Bulk conductivity formula:
Sample's electric resistance R (unit Ω) is calculated from potentiostat's voltage U (unit: V) and observed current I (unit: A) using Ohm's law: = Electric circuit of the experiment setup: Electrical resistivity ρ (unit: Ωm) is calculated from sample's resistance R, sample's length l (unit: m) and cross sectional area A (unit m 2 ). Junctions covered by the carbon glue are not counted to the crystal's length. Crystals are assumed to resemble cuboids so cross-sectional area A is width w (unit: m) times the height h (unit: m).