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Abstract

Magnesium aluminate MgAl2O4 (spinel) has many technological applications due to its
outstanding thermal, mechanical and optical properties. In order to obtain the desired
physical properties of spinel ceramics one of the main objectives is efficient control of
microstructure development during growth. A detailed study of (111) twins in natural
MgAl2O4 spinel crystals from Mogok (Myanmar) revealed that twinning is most probably
caused by the presence of beryllium (Be2+) during crystal growth, causing the formation of
local hcp stacking and the formation of twin boundary. Twinning in spinel is related to the
formation of intermediate polytypic compounds in the MgAl2O4-BeAl2O4 system.
In my thesis I demonstrated that twinning in spinel is in fact chemically induced and is
not a consequence of accidental attachment of crystals. In my study I focused on the
synthesis of spinel twins and related modulated structures in the BeO-MgO-Al2O3 ternary
system. For the flux-growth experiments I used a melt-forming agent PbF2. The chosen
growth conditions (1200°C, in air) were suitable for the synthesis of spinel, chrysoberyl and
related intermediate taaffeite structures. We have shown that the addition of small
amounts of BeO to the MgO-Al2O3 system results in abundant formation of spinel twins,
from simple contact twins to different combinations of twins, like parallel and cyclic twins,
resembling those found in nature. The twinning also has a dramatic effect on grain growth
in MgAl2O4-based ceramics causing anisotropic and exaggerated growth of spinel grains.
Higher additions of BeO lead not only to the abundant formation of {111} spinel twins,
but also to epitaxial growth of polytypic taaffeite layers on preexisting spinel grains.
HRTEM study of spinel-taaffeite intergrowths confirmed an orientation relationship of
[11̅0]·{111}Sp||[112̅0]·{0001}Taf. The basic structural element, observed on (111) twin
boundary in spinel where it is identical to the hcp sequence observed in ternary taaffeite
compounds (Be2+ replacing for Mg2+ on the interfacial tetrahedral interstices) and suggests
that twinning is probably the initial or a preparatory stage of the polytype formation. The
formation {111} twins infact trigger exaggerated grain growth in spinel ceramics. This
phenomenon can be exploited for the control of microstructure development in spinel
ceramics.
In the second part of my dissertation, I investigated the structure of (130) twins and rutile
precipitates in chrysoberyl from Bahia state of Brazil. The study is based on redefinition
of chrysoberyl space group and unit cell parameters that lead to proper determination of
twin phase. To provide the most reliable model for DFT and HRTEM simulations of the
twin boundary, space group 62 (Pmnb) was used. TEM study showed that the twin
boundary lies in (130) planes of the chrysoberyl structure enclosing an angle of ~59.5°
between crystal domains. DFT calculations were used for refinement of a rigid structural
model for (130) twin boundary. Besides twinning, where we determined local enrichment
with Ti, also numerous precipitates in chrysoberyl were noticed and described as TiO2 with
a distorted rutile structure that follows chrysoberyl orientation by
[001]·{120}Hr||[010]·{103}R. The enrichment with Ti at the twin boundary and the
formation of rutile-type TiO2 precipitates in bulk chrysoberyl can be understood as an
exsolution of Ti from a solid solution.