Corresponding author: Nina S. Kozlova ( kozlova_nina@mail.ru ) © 2020 Mark V. Weintraub, Nina S. Kozlova, Evgeniya V. Zabelina, Mikhail I. Petrzhik.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Weintraub MV, Kozlova NS, Zabelina EV, Petrzhik MI (2020) Effect of growth conditions on the mechanical properties of lanthanumgallium tantalate crystals. Modern Electronic Materials 6(2): 6570. https://doi.org/10.3897/j.moem.6.2.63731

The effect of growth conditions, anisotropy and polarity of specimens on the mechanical properties of lanthanumgallium tantalate La_{3}Ta_{0.5}Ga_{5.5}O_{14} single crystals grown in different atmospheres (argon (Ar), argon with oxygen addition (Ar+(<2%)O_{2} and Ar+(2%)O_{2}) and air) was studied. The test specimens for the measurements were cut perpendicularly to a 3^{rd} order axis (Z cuts) and in polar directions perpendicular to a 2^{nd} order axis (Y cuts). The polarity of the Y cut specimens was tested by piezoelectric response. The brittleness was evaluated by microindentation at 3, 5, 10 and 25 g loads. The brittleness proved to show itself at a 5 g and the higher loads regardless of growth atmosphere. Therefore microhardness tests were done at loads of within 3 g. The microhardness HV of the specimens was measured with an DM 8B Affri microhardness tester by Vickers methods. The hardness H, elastic modulus E and elastic recovery coefficient R were measured with a Berkovich pyramid on a CSM NanoHardness Tester using the instrumented indentation (nanoindentation) method. Growth atmosphere was shown to affect the mechanical properties of lanthanumgallium tantalate crystals: crystals grown in an oxygenfree argon atmosphere had the lowest microhardness, hardness, elastic modulus and elastic recovery coefficient. The lowest microhardness was detected in Z cut specimens regardless of growth atmosphere. The mechanical properties of polar Y cuts proved to be anisotropic: the microhardness, hardness, elastic modulus and elastic recovery coefficient of these cuts were lower for positive cuts than for negative ones regardless of growth atmosphere. Y and Z cut langatate specimens grown in argon with less than two percent oxygen exhibited strong elastic modulus and elastic recovery coefficient anisotropy.
langatate, single crystal, growth atmosphere, mechanical properties, microhardness test, instrumental indentation, microhardness, hardness, elastic modulus, elastic recovery coefficient, anisotropy
Improvement of the performance of piezoelectric devices requires prospective materials with a new gamut of properties including lanthanumgallium tantalate La_{3}Ta_{0.5}Ga_{5.5}O_{14} (langatate, LGT). Lanthanumgallium tantalate crystals are noncentrosymmetrical trigonal symmetry 32 (L_{3}3L_{2}) and hence they have piezoelectric properties, their piezoelectric moduli being d_{11} = 6.63 × 10^{12} C/N and d_{14} = 5.55 × 10^{12} C/N [
Polar LGT cuts are successfully used today as working components of piezoelectric devices [
To fabricate a sensing element for use in pressure gages one should mechanically treat langatate crystals (cutting, polishing and grinding). How there are but scarce data on the mechanical properties of langatate crystals [
Of greatest interest is the working polar cut perpendicular to a 2^{nd} order axis ((101–0) or (011–0)) but data on the microhardness of this cut are available in only one work [
The optical and electrical parameters of langatate are known to depend largely on crystal growth atmosphere [
Working piezoelectric cuts of langatate crystals are polar [
Thus there is the need for a systematic study of the mechanical properties of lanthanumgallium tantalate crystals including their polar cuts finding practical applications in sensing elements of hightemperature pressure gages.
Growth atmosphere  Anneal  Color  Cut  Direction  Microhardness, GPa  Ref. 

Ar+(1%) O_{2}  Vacuum  No  Y54º  –  12.2  [8] 
No  Orange  –  13.8  
–  –  –  (112–0)  [11–00]  7.7±0.1  [9, 10] 
–  –  –  [0001]  8.3±0.1  
–  –  –  (112–0)  –  10.22  [11] 
–  –  –  (101–0)  –  10.08  
–  –  –  (0001)  –  8.77 
Z cut and polar Y cut specimens grown in different atmospheres were studied: argon (Ar), argon with oxygen (Ar + < 2 % O_{2}), argon with oxygen (Ar + 2 % O_{2}) and air.
Plate side polarity was tested by piezoelectric response. Vickers microhardness (HV) was tested at a constant dwell time (10 s) and load advancing speed (50 mm/s) on an automatic microhardness tester DM 8B (Affri, Italy) allowing measurements at small loads (1, 3, 5, 10 and 25 g). The hardness H, indentation elastic modulus E_{I} and elastic recovery coefficient R were measured and anisotropy and effect of growth atmosphere on the mechanical properties of langatate crystals were studied with a Berkovich pyramid [
Hardness measurements have a large number of research and technical applications although there is still a discussion regarding the physical sense of this parameter and correct evaluation methods [
The basics of the methods being considered is the analytical solution [
For Vickers microhardness testing (GOST 299975) [
$HV=\frac{P}{M}=\frac{2P\mathrm{sin}\left(\frac{\alpha}{2}\right)}{{d}^{2}}=1,854\frac{P}{{d}^{2}}$ (1)
The conventional method of indenter and specimen interaction was justified for the case of negligibly small surface forces in comparison with the total interaction forces. For materials with a higher percentage of elastic deformation this method gives overestimated hardness [
The advantage of instrumental indentation is determined by the use of highprecision resolution for the depth of indentation and the magnitude of the applied load, which reach nanoscale values, namely nanometers and nanoNewtons [
Indentation size is measured for the maximum indentation depth h_{m} in the assumption that the diamond indenter is not deformed upon indentation. II curves are similar to tension curves at low deformations (Fig.
Typical experimentation II curves – family of 9 indentation curves of sample 2. Fn – applied load, mN, Pd – penetration depth, nm.
Hardness is calculated as the ratio of the maximum load to the unrecovered indentation projection area and elastic modulus is determined based on the indentation area and the contact stiffness as S = dP/dh from the slope of the unloading curve upper third portion.
$E=\frac{S}{2}{\left(\frac{\mathrm{\pi}}{{A}_{p}}\right)}^{\frac{1}{2}}$ (2)
where E is Young’s modulus, n is Poisson’s ratio of the tested material, h_{с} is depth over which the indenter and specimen are in contact during the force application, A_{p} is projected (cross section) area of indenter h_{с}. Indenter penetration into material produces a complex stressed state in the vicinity of the contact area which is close to uniform compression, the indepth propagating deformation having elastic (recoverable) and plastic (nonrecoverable) components. This allows II to be used for retrieving information on hardness, Young’s modulus and elastic recovery coefficient in total deformation characterized by elastic recovery
$R=\frac{{h}_{\mathrm{max}}{h}_{p}}{{h}_{\mathrm{max}}}$, (3)
where h_{max} is maximum value of h, h_{p} is the permanent recovered indentation depth after removal of test force.
Preliminary microhardness tests of Y cut langatate crystals grown in an Ar + < 2 % O_{2} atmosphere were conducted. Langatate is a brittle crystal and therefore the indentation loads were small: 3, 5, 10 and 25 g. Figure
Indentation photos for different loads in Y cut langatate specimen grown in an Ar + < 2 % O_{2} atmosphere: (a) 3 g, (b) 5 g, (c) 10 g and (d) 25 g
With 3 g load indenter produces a clear imprint without visible cracks or cleaves (Fig.
The brittleness of the material was evaluated following the method described earlier [29] on a fivepoint scale where each of the indentations is given a brittleness rate determined on an arbitrary scale (Table
Mean brittleness rate  Indentation pattern 

0  Indentation without visible cracks or cleaves 
1  One small crack at indentation corner 
2  One crack not coincident with indentation diagonal extension. Two cracks in adjacent indentation corners 
3  Two cracks in opposite indentation corners. Three cracks in different indentation corners. Cleave at one indentation side 
4  More than three cracks. Cleaves at two indentation sides 
5  Complete indentation shape destruction 
Tracking the development of cracks during microindentation of a langatate crystal and evaluating its brittleness one can conclude that the brittleness of the material starts to show itself at a 5 g load. These results suggested that Vickers microhardness testing of langatate crystals requires loads of within 3 g.
The effect of growth atmosphere on langatate microhardness was studied for a 3 g indenter load. Figure
Indentations in langatate crystal grown in an Ar + < 2 % O_{2} atmosphere for 3 g load: (a) Y cut (+), (b) Y cut (–) and (c) Z cut
Growth atmosphere  Microhardness, GPa  

+  –  
Ar + 2 % O_{2}  6.5 ± 5 %  7.7 ± 5 % 
Ar + < 2 % O_{2}  6.5 ± 5 %  7.0 ± 5 % 
Ar  5.9 ± 5 %  7.4 ± 5 % 
Air  6.5 ± 5 %  7.7 ± 5 % 
Table
Langatate crystal microhardness measurement data (growth atmosphere Ar + < 2 % O_{2}) for different cuts.
Cut  3 g load microhardness, GPa 

3 g  
Y cut (+)  6.5 ± 5 % 
Y cut (–)  7.0 ± 5 % 
Z cut  5.9 ± 5 % 
Thus Vickers microhardness measurements show langatate crystal microhardness to depend on growth atmosphere. Crystals grown in an oxygen containing atmosphere have a higher microhardness than those grown in an argon atmosphere. Langatate crystal microhardness exhibits anisotropy regardless of growth atmosphere: the microhardness of positive polarity sides for polar cuts is lower than that of negative polarity sides, the Z cut having the lowest microhardness.
Since langatate crystals are brittle (Fig.
The II test results (Table
II results on anisotropy of mechanical properties in langatate crystals are summarized in Table
The results confirm the conclusions made for Vickers hardness tests: the hardness of positive polarity cuts is lower than that of negative ones. The difference between the Vickers and II hardness data can be accounted for the possible reasons: either the cracks occur, but not visible, or the surface layer is harder then the deep layer due to the polishing characteristics. Thus, this requires further research.
Appearance of Berkovich indentation in Y cut (–) langatate crystal specimen grown in an argon atmosphere
Mechanical properties data obtained by instrumented indentation for langatate crystals, grown at different conditions.
Growth atmosphere  Hardness, GPa  Elastic modulus, GPa  Elastic recovery coefficient, %  

+  –  +  –  +  –  
Ar  10.7 ± 1%  11.0 ± 1%  142 ± 3%  143 ± 3%  39 ± 10%  40 ± 10% 
Ar + <2%O_{2}  11.9 ± 1%  12.1 ± 1%  147 ± 3%  148 ± 3%  41 ± 10%  41 ± 10% 
Air  12.1 ± 1%  12.6 ± 1%  146 ± 3%  147 ± 3%  42 ± 10%  43 ± 10% 
Ar+2%O_{2}  12.0 ± 1%  12.0 ± 1%  146 ± 3%  148 ± 3%  42 ± 10%  42 ± 10% 
Mechanical properties data obtained by instrumental indentation for different cuts of langatate crystals grown in an Ar + < 2 % O_{2} atmosphere.
Cut  Hardness, GPa  Young’s modulus, GPa  Elastic recovery coefficient, % 
Y cut (+)  11.9 ± 1 %  147 ± 3 %  41 ± 10 % 
Y cut ()  12.6 ± 1 %  146 ± 3 %  41 ± 10 % 
Z cut  12.1 ± 1 %  183 ± 3 %  35 ± 10 % 
The brittleness of langatate crystals manifests itself at loads of 5 g and higher regardless of crystal growth atmosphere.
Growth atmosphere has an effect on the mechanical properties of langatate. The microhardness, hardness, elastic modulus and elastic recovery coefficient are higher for crystals grown in an oxygen containing atmosphere.
The microhardness of polar Y cut langatate crystals exhibits anisotropy. The microhardness, hardness and elastic modulus of the positive polarized side of Y cut is lower than for negative one. The microhardness of Y cut langatate crystal is higher than that of Z cut one.
Instrumented indentation data suggest that Z and Y cut langatate crystals grown in argon with less than two percent oxygen exhibit strong elastic modulus and elastic recovery coefficient anisotropy.
The Authors are grateful to JSC FomosMaterials and Oleg A. Buzanov for providing lanthanumgallium tantalate crystals.