Cryst. Res. Technol. 44, No. 6, 577 – 580 (2009) / DOI 10.1002/crat.200900032
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
A new piezoelectric single crystal obtained by Ge doping in the
SiO
2
structure
M. Miclau*
1
, N. Miclau
2
, M. Poienar
1
, and I. Grozescu
1
1
Condensed Matter Research Department, National Institute R&D Electrochemistry and Condensed Matter,
Plautius Andronescu 1, Timisoara, Romania
2
Politehnica” University of Timisoara, Bd. Vasile Parvan 2, Timisoara, Romania
Received 19 January 2009, revised 8 March 2009, accepted 12 March 2009
Published online 3 April 2009
Key words alpha-quartz, hydrothermal, piezoelectric.
PACS 81.10.-h, 77.84.-s
The interest of Si
1-x
Ge
x
O
2
single crystals with alpha-quartz structure is connected to improvement of
electromechanical coefficients and rise of α – β phase transition of quartz one. Growth of an α -Si
x
Ge
1-x
O
2
crystal was realized by a hydrothermal method of temperature gradient in autoclaves, made from Cr–Ni
alloys. Nutrient material was prepared from synthetic quartz as crashed rods and placed in the bottom of
autoclaves. There was loaded GeO
2
powder additive in proportions to quartz nutrient. Single crystals were
investigated by electron microprobe analysis, X-ray diffraction and atomic force microscopy. The most
important result, which was obtained during the investigations, is an experimental proof of growth of α-
Si
x
Ge
1-x
O
2
single crystals under the hydrothermal conditions. The present results thus open the possibility to
tune the piezoelectric properties of these materials by varying the chemical composition.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
α-Quartz type materials are of major interest due to their piezoelectric properties. In addition to α-quartz itself,
this family is composed of GeO
2
and compounds with the general formula A
III
B
V
O
4
where A= Al, Ga, Fe, B,
and B=P and As.
The structural distortion present can be described by two interrelated angles: the intertetrahedral bridging
angle A-O-B, θ, and the tetrahedral tilt angle δ [1]. In the case of undistorted tetrahedra, the relation between
these two parameters is given by:
2
3 1
cos cos
4 2 3
θ δ
= - +
(1)
For very well characterized crystals (SiO
2
, AlPO
4
and GaPO
4
), it proved possible to relate linearly their A-O-B
values (143.7°, 142.8° and 134.6° respectively) to their piezoelectric properties [2]. The more distortion
structure, the more the θ and δ values decrease and increase, respectively. One of main way to obtain a more
distortion structure, it is to increase the average size of ‹r
A
› of A-site cations.
Therefore, in the present paper, the Si
1-x
Ge
x
O
2
single crystal, with x up to 0.30 were obtained by
hydrothermal method and the crystal structure was determined by X-ray diffraction. These results can be used
to tune the piezoelectric properties of these materials by varying the chemical composition.
____________________
* Corresponding author: e-mail: marinela.miclau@gmail.com