1. Introduction
Piezoelectric materials that can operate under very high temperature without degradation are sought for the control of structure materials and control system in turbines, engines, nuclear reactors,
etc. [
1,
2]. However, the use of a piezoelectric material at elevated temperature presents many challenges such as possible phase transition, chemical degradation or structural defect propagation which can cancel or lead to instability of the piezoelectric properties.
Non-pyroelectric single crystals with the α-quartz-like structure exhibiting both higher piezoelectric constants and a higher thermal stability as compared to α-quartz would be promising materials to build miniaturized high temperature piezoelectric-operated devices without cooling. In the XO
2 (X = Si, Ge) and MPO
4 (M = Fe, Al, Ga, B) family, the α-quartz-like structure is composed of either only XO
4 corner-shared tetrahedra or of both MO
4 and PO
4 tetrahedra forming a trigonal system [
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16]. The α-phase (
P3
121 or
P3
221, respectively left-handed or right-handed,
Z = 3) is derived from the β-phase (
P6
222 or
P6
422) by a symmetry loss induced by a tilt of the tetrahedra around the
b-axis. Two structural distortion parameters exist to quantify this phenomenon: the average intertetrahedral bridging angle θ (X–O–X or M–O–P) and the tetrahedral tilt angle δ [
11,
14,
17,
18,
19,
20]. Based on experimental characterizations and theoretical studies on α-quartz analogues, it was demonstrated that the higher piezoelectric properties of α-GaPO
4 and α-GeO
2 materials compared to α-SiO
2, α-FePO
4 and α-AlPO
4 compounds were directly related to their structural distortion with respect to the β-quartz structure type (for β-SiO
2 θ = 153.3° and δ = 0° at 575 °C) [
11,
14,
17,
21,
22,
23,
24,
25,
26,
27].
In addition, the well-known α-β phase transition, which appears around 573 °C in α-SiO
2 (θ = 144.2°) [
19,
26] does not occur when the tilt angle δ is over 22° (leading to θ under 136°) [
9,
13,
14,
22,
25]. In other words, the transition from an α phase to a β phase is absent for α-GeO
2 and α-GaPO
4 crystallized materials (θ = 130.04° and θ = 134.60°, respectively) [
13,
14,
17,
22,
28,
29].
Under ambient conditions, GeO
2 exhibits two persistent forms of differing anion coordination around the central cation: the α-quartz-type modification (trigonal) with 4:2 coordination [
16] and the rutile-type modification (tetragonal
P4
2/mnm,
Z = 2) with 6:3 coordination [
4,
11,
30,
31,
32,
33,
34]. Naturally occurring, GeO
2 is known to be more stable in the rutile structure than in the α-quartz structure. The transformation from the rutile-like form to the α-quartz-type form has been reported to occur under normal atmospheric pressure in the 1024–1045 °C temperature range [
11,
35]. The kinetics of the allotropic transformation from a trigonal to a tetragonal structure is extremely low, and non-appreciable without the presence of any catalyst such as traces of GeO
2 rutile-like form, chlorides or water [
11,
36,
37,
38,
39].
A transition from the thermodynamically stable α-quartz-like GaPO
4 phase to the β-cristobalite modification is known to occur above 960 °C [
13,
40,
41,
42,
43,
44,
45,
46]. This high temperature allotropic transition does not permit the growth of α-GaPO
4 crystals by conventional melt techniques. Single crystals of α-GaPO
4 were grown using epitaxial hydrothermal-based methods in a retrograde-solubility range at
T < 250 °C and in the range of direct temperature dependence of solubility at
T > 300 °C from highly corrosive acid solutions [
40,
43,
47,
48,
49,
50,
51,
52,
53,
54,
55,
56,
57,
58,
59,
60,
61,
62]. However, for gallium orthophosphate material, it seems quite difficult to obtain very high quality crystals with these solution-based growth methods. The presence of twins, dislocations and/or a quite high level of hydroxyl group incorporated via the growth medium tend to deteriorate the piezoelectric properties especially at high temperatures as physical properties are very sensitive to material perfection [
43,
49,
50,
57,
63,
64,
65,
66,
67,
68,
69,
70,
71,
72,
73]. The OH impurities in α-GaPO
4 single crystals are responsible for the “milky” appearance of the samples when exposed to high temperature (600 °C). This typical behavior is due to water trace precipitation effects and corresponds to the upper temperature limit of physical property measurements [
65,
69,
74,
75].
For GeO
2, the temperature region of existence of the stable trigonal phase goes from 1033 °C up to the melting point of 1116 °C [
25,
30,
76]. When epitaxially-grown in the temperature region where it is metastable, the hydrothermally-grown alpha-quartz form of GeO
2 contains high OH impurities which rather easily catalyze its return to the thermodynamically stable rutile-like structure when heated as low as 180 °C [
77,
78,
79,
80,
81].
In this context, it appeared that another suitable growth technique for oxides could be applied for the crystallization of α-GaPO
4 and α-GeO
2 materials to get single crystals with a high degree of structural and chemical perfection; the high temperature solution growth technique also known as the fluxed melt growth [
82,
83,
84,
85]. Using molten inorganic salts, or flux, playing the role of solvent, crystals of a compound can be obtained below its melting or transformation point. The principle of flux growth is based on the spontaneous nucleation that occurs when a supersaturation is obtained either upon cooling of a high temperature solution or by boiling away a volatile solvent at a fixed temperature [
86]. The fluxed melt method presents some advantages compared to hydrothermal-based growth methods: the material can be crystallized at atmospheric pressure and the inorganic solvents used are water-free at high temperature.
This paper presents an overview of the main results obtained from several structural and physical characterizations undertaken on flux-grown α-GaPO4 and α-GeO2 piezoelectric crystals. When possible, the results are compared and discussed in the view of hydrothermally-grown α-quartz-like SiO2, GeO2 and GaPO4.
6. Conclusions
In the family of α-quartz isotypes, high quality α-GaPO4 and α-GeO2 single crystals with a larger thermal stability and higher piezoelectric properties than α-quartz can potentially be used as piezoelectric materials for high temperatures applications. However if these single crystals are grown via hydrothermal-based methods, the presence of significant numbers of hydroxyl groups and structural defects in their network lead to a degradation of their physical properties and even to a phase transformation at relatively low temperature for α-GeO2.
The main structural and chemical results obtained from infrared studies (OH-content), thermal behavior analysis and Brillouin scattering experiments on MoO3-based flux-grown α-GaPO4 and α-GeO2 single-crystals have demonstrated OH-free high quality piezoelectric crystals and an improved thermal stability.
These results confirm on one hand the high potential of flux-grown α-GaPO4 and α-GeO2 single crystals as piezoelectric materials for high temperature applications and on the other hand a very powerful method of high temperature flux melted technique to grow high quality α-quartz isotype single crystals.
Moreover, as iso-structural to α-quartz, most of the device designs developed for α-quartz single crystal could be applied for α-GeO2 with minor adaptations. The potential of α-GeO2 single crystal for the realization of piezoelectric devices is also confirmed as its d11 piezoelectric constant at ambient temperature is found to be more than twice that of α-quartz. The piezoelectric property of α-GeO2 is still conserved at high temperature as a significant piezoelectric contribution to C11 still exists at 1000 °C.