Corresponding author: Evgeniya V. Zabelina (zabev@mail.ru)
Single crystal calcium molybdate CaMoO4 is a well-known material. However the interest to CaMoO4 has recently grown due to a number of its important applications including as a working material in cryogenic scintillation bolometers. CaMoO4 single crystals acquire blue color during growth due to the presence of color-center type defect centers which are unacceptable for optical applications. Color can be eliminated through annealing in an oxygen containing atmosphere, following which required optical components can be produced from the single crystals by mechanical treatment (cutting, polishing etc.). Therefore assessment of the mechanical properties of these single crystal materials is an important task for the optimal solution of issues occurring in the fabrication of optical components and their further practical application. There are but scarce data on the mechanical properties of CaMoO4, and the available ones have been reported without allowance for anisotropy. There is a significant scatter of data on the Mohs hardness of the single crystals, ranging from 3.3 to 6 in different publications. In this work we present data on calcium molybdate single crystals in the initial state and after high-temperature anneals of different durations in an oxygen containing atmosphere. We show that long-term annealing leads to decolorization of the crystals. Calcium molybdate single crystals prove to be quite brittle: the brittleness index
Zabelina EV, Kozlova NS, Buzanov OA, Krupnova ED (2023) Effect of post-growth anneals in oxygen-containing atmosphere on the microhardness of single crystal calcium molybdate CaMoO4.
Synthetic calcium molybdate (powellite) CaMoO4 is a well-known material. The first works dealing with this material date back to the 1940s [
CaMoO4 crystal lattice parameters
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1 | 5.213 | 11.395 | [2] |
2 | 5.222 | 11.425 | [5] |
3 | 5.226 | 11.430 | [6] |
4 | 5.349 | 12.020 | [3] |
Single crystal calcium molybdate was initially used in tunable acousto-optic filters [
Along with the requirement to transparency in the working wavelength range and other requirements, calcium molybdate crystals are also expected to exhibit appropriate mechanical properties [
Hardness is defined as crystal’s resistance to cutting, scratching or indentation. The numerical unit of hardness is the ratio between the indentation load and the dimensions of the indentation produced by the indenter, or the width or length of the scratch produced on the crystal’s face [
Anisotropic media to which calcium molybdate refers may exhibit I and II type hardness anisotropy. I type anisotropy is polar anisotropy which depends on indenter axis direction relative to the crystallographic directions of the crystal face, whereas II type anisotropy implies different microhardness for different crystal faces [
Calcium molybdare microhardness data are quite scarce in literature and are presented without allowance for anisotropy or specification of indentation loads applied. Furthermore, the data available have a noticeable scatter (Table
CaMoO4 hardness data
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1950 | Not Specified | 3.5 | [21] |
2001–2005 | Not Specified | 3.5–4 | [5] |
2008 | Not Specified | 4 | [2] |
1988 | (011) | 5.3 | [22] |
(001) | 4.8 | ||
2002 | Not Specified | 6 | [23] |
The aim of this work is to study the effect of isothermal anneals in an oxygen containing atmosphere on the microhardness of CaMoO4 single crystals and its anisotropy.
The CaMoO4 crystals were grown at FOMOS-Materials JSC with the Czochralsky technique from stoichiometric charge with an addition of an excess of МоО3 in Pt crucibles on a Kristall-3M growth set up with RF heating. The crystals were cut into oriented 10 × 10 × 10 mm dice with the
The specimens were studied as-grown and as-annealed for different time (6 and 1000 h) in an oxygen containing atmosphere at
Calcium molybdate crystal specimens: (
In this work, the hardness was tested by the Vickers method (Vickers Hardness, HV) [
The Vickers hardness is proportional to the indentation load divided by the indentation base surface area, the latter being calculated from the lengths of its diagonals in the assumption that the indentation has a regular pyramid shape with a square base and the vertex angle equal to the indenter’s vertex angle, and is calculated using the following equation [
where
The microhardness was measured at the Inter-University Test Laboratory for semiconductors and dielectrics “Single Crystals and Stock on their Base” on a calibrated Aaffri DM 8 B microhardness meter allowing indentation testing at small loads, beginning from 1 g. The dwell time was 10 s, the load rate being 30 mm/s. The microhardness was calculated automatically from the measurements of the indentation diagonals using a CCD camera with the Hardtest-Program Precidure Ver. 2.4 software of Frits Mueller GmbH. HV measurement correctness was controlled directly before specimen testing by measuring the hardness of the lithium fluoride (LiF) reference specimen produced and tested at the Inter-University Test Lab. The measurement accuracy was not worse than 5%.
All the specimen surfaces were indented using the same method taking into account crystal orientation (Fig.
Schematic of indentations on (
The measurements were carried out at 1, 3, 5, 10 and 25 g loads.
For a 1 g load the indentations were irresolvable regardless of specimen type. For a 3 g load the indentations were surrounded by cleaves and cracks for all the specimens regardless of crystal face orientation. This observation suggests that the CaMoO4 crystals are extremely brittle.
Typical calcium molybdate microhardness indentations are shown in Figs
Typical indentations on the
Typical indentations on the
For understanding the complete indentation destruction evolution the indentation load was increased to the limit indentation destruction load
– 10 g for the
– 25 g regardless of face orientation for the light-blue specimen annealed for 6 h;
– 25 g for the
Isothermal oxygen annealing increased
The as-annealed specimens were less brittle than the as-grown ones. The high brittleness of materials us typically attributed to the fact that the load exerted by the diamond pyramid during indentation exceeds the limit strength of the material [
For numerical evaluation of the material’s brittleness, the brittleness index
The brittleness index of the specimens was evaluated using the method described earlier [
Material brittleness evaluation [27]
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0 | Indentation without visible cracks or cleaves |
1 | One minor crack at indentation corner |
2 | One crack not aligned with indentation diagonal. Two cracks at adjacent indentation corners |
3 | Two cracks at opposite indentation corners. Three cracks at different indentation corners. Cleave at one indentation side |
4 | More than three cracks. Cleaves at two indentation sides |
5 | Complete destruction of indentation shape |
The average brittleness index of the CaMoO4 specimens as determined in accordance with the abovementioned approach [
Palmqvist toughness factor evaluation [
Since cracks occurred at each of the indentations, the ductility was calculated for all the loads used.
For the specimens whose microhardness could be measured for different loads, an increase in the microhardness either remains the same within the experimental error with a change in the ductility (
Vacuum annealing increases the ductility of all the test specimens regardless of cut orientation.
The resultant HV microhardness was converted to Mohs hardness HM since in the literary available publications [
The microhardness data are close to the earlier literary ones [
All the test specimens exhibited II type microhardness anisotropy: the microhardness of the
The anisotropy coefficient KH is used to evaluate the anisotropy degree for anisotropic materials, its formula being as follows [
where
Due to the brittleness of the crystals the microhardness anisotropy coefficient was determined in accordance with Eq. (4) for different loads and yielded as follows:
– 1.064 for the initial blue specimen (load 3 g);
– 1.061 for the annealed light-blue specimen (load 10 g);
– 1.02 for the annealed colorless specimen (load 25 g).
Microhardness measurements allow evaluating the bond ionicity degree in accordance with the following earlier formula [
HM =
where
The closer the resultant
It was shown [
The bond ionicity degree calculated using Eq. (6) for the test crystals proved to be higher for the
The HV and HM microhardness data, the bond ionicity degree
Parameters of CaMoO4 specimens taking into account anisotropy in the initial state and after isothermal anneals in an oxygen containing atmosphere
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Load | 3 g | 5 g | 5 g | 10 g | 5 g | 10 g | 25 g | |
HV, kgf/mm2 | 410±20 | 460±20 | 340±20 | 450±20 | 390±20 | 380±20 | – | |
HM | 5.0 | 5.2 | 4.7 | 5.2 | 4.9 | 4.9 | – | |
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0.90 | 0.89 | 0.92 | 0.89 | 0.91 | 0.91 | – | |
10 (98.07) | 25 (245.2) | 25 (245.2) | ||||||
0.52 | 1.04 | 1.21 | 2.98 | 1.13 | 3.26 | – | ||
HV, kgf/mm2 | 350±20 | – | – | 380±20 | – | 360±20 | 410±20 | |
HM | 4.7 | – | – | 4.9 | – | 4.8 | 5.0 | |
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0.92 | – | – | 0.91 | – | 0.91 | 0.90 | |
5 (49.03) | 25 (245.2) | 50 (490.3) | ||||||
0.56 | – | – | 3.26 | – | 3.34 | 12,30 | ||
KH | 1.064 | – | – | 1.061 | – | – | 1.021 | |
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5 | 4 | 4 |
First CaMoO4 microhardness data were obtained for specimens in the initial as-grown state and after high-temperature anneals in an oxygen containing atmosphere. CaMoO4 specimens are quite brittle, cracks and cleaves occurring in the vicinity of indentations at such a small load as 3 g for all the test specimens. Limit indentation destruction loads
Crystal brittleness was characterized with brittleness indices
The experimental Mohs microhardness figures HM are ~5 for the
Based on the microhardness data the bond ionicity degree was determined for the CaMoO4 specimens in the initial state and after isothermal anneals in an oxygen containing atmosphere.
The experiments were carried out with financial support under State Assignment FSME-2023-0003 at the Inter-University Test Laboratory for semiconductors and dielectrics “Single Crystals and Stock on their Base” of the National University of Science and Technology MISIS.