A pyroxene with composition LiNiSi
2O
6 was synthesized at
T = 1,473 K and
P = 2.0 GPa; the cell parameters at
T = 298 K are
a = 9.4169(6) Å,
b = 8.4465(7) Å,
c = 5.2464(3) Å,
β = 110.534(6)°,
V = 390.78(3) Å
3. TEM examination of the LiNiSi
2O
6 pyroxene showed the presence of
h +
k odd reflections indicative of a primitive lattice, and of antiphase domains obtained by dark field imaging of the
h +
k odd reflections. A HT in situ investigation was performed by examining TEM selected area diffraction patterns collected at high temperature and synchrotron radiation powder diffraction. In HTTEM the LiNiSi
2O
6 was examined together with LiCrSi
2O
6 pyroxene. In LiCrSi
2O
6 the
h + k odd critical reflections disappear at about 340 K; they are sharp up to the transition temperature and do not change their shape until they disappear. In LiNiSi
2O
6 the
h + k odd reflections are present up to sample deterioration at 650 K. A high temperature synchrotron radiation powder diffraction investigation was performed on LiNiSi
2O
6 between 298 and 773 K. The analysis of critical reflections and of changes in cell parameters shows that the space group is
P-centred up to the highest temperature. The comparative analysis of the thermal and spontaneous strain contributions in
P2
1/
c and
C2/
c pyroxenes indicates that the high temperature strain in
P-LiNiSi
2O
6 is very similar to that due to thermal strain only in
C2/
c spodumene and that a spontaneous strain contribution related to pre-transition features is not apparent in LiNiSi
2O
6. A different high-temperature behaviour in LiNiSi
2O
6 with respect to other pyroxenes is suggested, possibly in relation with the presence of Jahn–Teller distortion of the M1 polyhedron centred by low-spin Ni
3+.
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