Synthesis and characterisation of lanthanum germanate-based apatite phases

Synthesis of hexagonal lanthanum germanate apatites through site selective isovalent doping with yttrium

“…In this paper we confirm that such site selective doping is successful, and does indeed lead to the formation of hexagonal apatites even for high oxygen contents. can be related to the fact that in this temperature range, even the undoped La 9.33+z (GeO 4 ) 6 O 2+3z/2 samples are hexagonal; prior reports showing that there is a change from a triclinic to a hexagonal cell at high temperatures [7,16]. Thus the enhanced lower temperature conductivity can be correlated with the more facile oxide ion conduction within the higher symmetry hexagonal lattice.…”

“…In terms of the former it is the Lanthanum silicate and germanate materials, La 9.33+z (Si/GeO 4 ) 6 O 2+3z/2 which have attracted considerable attention due to their high oxide ion conductivities at elevated temperatures [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In contrast to the traditional fluorite and perovskite-type oxide ion conductors, where conductivity is mediated by oxide ion vacancies, the weight of experimental and theoretical evidence for the apatite systems indicates that they conduct via an interstitial mechanism.…”

Cooperative oxide-ion conduction in apatite-type lanthanum germanate—A first principles study

“…Abram et al measured the conductivities of the lanthanum germanate at two compositions [x = 0, 0.34 in La 9.33+x (GeO 4 ) 6 O 2+x ] containing different concentrations of La vacancies. 25) These show comparable conductivities and apparent activation energies around 0.8 eV in the high-temperature range. The reported activation energies are slightly higher than that in the present study, which is probably attributed to the change in local environments around La vacancies.…”

Apatite germanates doped with tungsten: synthesis, structure, and conductivity

“…However, for x>0.7, the apatite symmetry is reduced from hexagonal to triclinic, with a coincident depression of conductivity at low temperatures that is attributed to more efficient trapping of the interstitial oxygen [24]. The same transition to a triclinic cell is observed for the La9.33+2x/3Ge6O26+x series at high values of x [7,10,20,21]. Therefore it is important to identify strategies to maintain hexagonal symmetry at high oxide ion contents to maintain high conductivities.…”