Solid State Chemistry @Aalto
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Introduction

 Lanthanum oxide (La2O3) is a p-type semiconductor with the largest band gap among rare earth oxides (4.3 eV),  high dielectric constant (ε = 27) 

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M.R. Sovizi, S. Mirzakhani, A chemiresistor sensor modified with lanthanum oxide nanoparticles as a highly sensitive and selective sensor for dimethylamine at room temperature, New J. Chem., 44 (12) (2020), pp. 4927-4934,https://doi.org/10.1039/C9NJ06329C

and low lattice energy. With these properties, La2O3 material has been studied in many application areas as batteries, magnetic storage, biosensors, catalysts, dielectric layers, optical coating, and photoelectric conversion.

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S. Karthikeyan, K. Dhanakodi, S. Surendhiran, K.S.G. Jagan, P. Thirunavukkarasu, L. Arunraja, Effect of synthesis parameters on the structural, morphological characteristics, and photocatalytic activity of La2O3 nanoparticles, Journal of the Indian Chemical Society, Volume 100, Issue 1, 2023, 100860, https://doi.org/10.1016/j.jics.2022.100860


There are four different crystal structure of La2O3, C-type, A-type, H-type and X-type, the transformation of these structures are depending on the temperature. 

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Gin-ya Adachi and Nobuhito Imanaka, The Binary Rare Earth Oxides, Chemical Reviews 1998 98 (4), 1479-1514, https://doi.org/10.1021/cr940055h


C-type(Ia-3) is considered to be the structure with lowest energy and can transform to A-type(P-3m1) around 870K

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Mehrotra, P. N., Chandrashekar, G. V., Rao, C. N. R., & Subbarao, E. C. Phase transformations of rare earth sesquioxides La2O3, Nd2O3, Pr2O3 and Y2O3 . Trans. Faraday Soc., 1966,62, 3586-3590 . https://doi.org/10.1039/TF9666203586


H-type(P63/mmc) and X-type(Im-3m) were the high temperature phases of La2O3. The transformation from A-type to H-type occurs around 2300K and can be further transformed to X-type around 2373K.

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P. Aldebert, J.P. Traverse, Etude par diffraction neutronique des structures de haute temperature de La2O3 et Nd2O3, Materials Research Bulletin, Volume 14, Issue 3, 1979, Pages 303-323, https://doi.org/10.1016/0025-5408(79)90095-3

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Table 1. Basic information of La2O3

Appearancewhite, dry powder
Melting temperature2529K
Boiling temperature3893K
Lattice energy−12687 kJ/mol
Standard Gibbs free energy−1784.833 kJ/mol
Density in room temperature6.28 g/cm³
Molar mass325.7974 g/mol




C-type La2O3

The C-type structure is the thermodynamically most stable structure, however it was discovered after the A-type structure. It because the temperature of transformation from C-type to A-type is just below the decomposition temperature of La(OH)3 or other compounds of lanthanum.

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Synthesis

Obtain amorphous La2O3 from La(OH)3 decompose under vacuum condition (~10-5 torr). C-type structure crystal can be obtained by heating up the amorphous La2O3 to 673K around 4 hours.

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Structure illustration


Figure 1. Left: Crystal structure of C-type La2O3 (La ions in green and O ions in red). Right: Simulated XRD powder pattern of the structure. (generated with VESTA with data from ref. 6)(Figure: Xueran Tao)




Table 2. Structural information of C-type La2O3 

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Felsche, J. A new form of La2O3 . Naturwissenschaften 56, 212 (1969). https://doi.org/10.1007/BF01166817, 

ICSD Coll. Code7795
Space groupIa-3
Crystal system

cubic

Unit cell parameters:

a =  11.3270 Å 

b =  11.3270 Å

c =  11.3270 Å 

α = 90.0°

β = 90.0°

γ =90.0°

V = 1453.2645 Å^3



A-type La2O3

The A-type structure is another room-temperature structure of La2O3, and it sometime just been called as hexagonal phase in many articles. It is also the earliest synthesized structure among the 4 different structures of La2O3 because of its simple synthesis route. Also, it cause this structure to be the most commonly used in the applications of La2O3.

Synthesis

There are several different method to synthesize A-type La2O3, such as mechanochemical, sol-gel, solution combustion, and thermal decomposition. 

For sol-gel method, dissolve Lanthanum (III) nitrate hexahydrate (La (NO3)3·6H2O) in water around 333K to get transparent solution. Then add Ammonium hydroxide (NH4OH) to the solution until its pH reach to 10. Then sonicated the solution for 30 min at 40Hz power with a probe sonication to form A-type La2O3 nano-particle.

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Structure illustration


Figure 2. Left: Crystal structure of A-type La2O3.(La ions in green and O ions in red) Right: Simulated XRD powder pattern of the structure. (generated with VESTA with data from ref. 7)(Figure: Xueran Tao)





Table 3. Structural information of A-type La2O3 

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C. Gökhan Ünlü, M. Burak Kaynar, Telem Şimşek, Atakan Tekgül, Bora Kalkan, Şadan Özcan, Structure and magnetic properties of (La1−xFex)FeO3 (x = 0, 0.25, 0.50) perovskite, Journal of Alloys and Compounds, Volume 784, 2019, Pages 1198-1204, https://doi.org/10.1016/j.jallcom.2019.01.047

ICSD Coll. Code7795
Space groupP-3m1
Crystal system

trigonal

Unit cell parameters:

a =  3.98240 Å 

b =  3.98240 Å

c =  6.27190 Å 

α = 90.0°

β = 90.0°

γ =120.0°

V = 86.1429 Å^3



 H-type La2O3

The H-type structure is the first high temperature structure of La2O3 and the transformation occurs around 2300K. It has very similar XRD pattern than A-type. Even thought this structure has been found in 1979

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, the studies around this is quite rare and has not yet establish any commercial application. It is probably due to the high temperature requirement and the narrow stable temperature range(2300K-2373K).

Structure illustration


Figure 3. Left: Crystal structure of H-type La2O3 (La ions in green and O ions in red, white pattern is the vacancy which means oxygen occupies only 50%) Right: Simulated XRD powder pattern of the structure. (generated with VESTA with data from ref. 5)(Figure: Xueran Tao)





Table 4. Structural information of H-type La2O3 

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ICSD Coll. Code100209
Space groupP63/mmc
Crystal systemhexagonal
Unit cell parameters:

a =  4.057 Å 

b =  4.057 Å

c =  6.430 Å   

α = 90.0°

β = 90.0°

γ = 120.0°

V = 91.6540 Å^3



X-type La2O3

The X-type is the other high temperature structure of La2O3. The transformation from H-type to X-type occurs around 2370K. It is the last structure La2O3 of before melting. This structure was published together with H-type in the same paper

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. Same than H-type structure, the application has not been established due to the high temperature requirement and the narrow stable temperature range before melts(2373K-2529K).

Structure illustration


Figure 4. Left: Crystal structure of X-type La2O3 (La ions in green and O ions in red white pattern is the vacancy which means oxygen occupies only 50%) Right: Simulated XRD powder pattern of the structure. (generated with VESTA with data from ref. 5)(Figure: Xueran Tao)





Table 5. Structural information of X-type La2O3 

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ICSD Coll. Code44692
Space groupIm-3m
Crystal systemCubic
Unit cell parameters:

a =  4.510 Å  

b =  4.510 Å

c =  4.510 Å   

α = 90.0°

β = 90.0°

γ = 90.0°

V = 91.7339 Å^3



Application

The application of La2O3 has been well established since La2O3 has been used in in dielectric layers, optical coating, and capacitor additive, but they uses majorly A-type structure as it has a better crystalline nature, better temperature stability, higher dielectric constant, and lower leakage current.

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Optical glass with addition of La2O3 provide higher visible region and the optical bandgap.

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A. Masuno, H. Inoue, K. Yoshimoto, and Y. Watanabe, "Thermal and optical properties of La2O3-Nb2O5 high refractive index glasses," Opt. Mater. Express  4, 710-718 (2014). doi.org/10.1364/OME.4.000710


Barium strontium titanite glass–ceramics(BST) is one of the candidates  to achieve high energy-storage density. The addition of La2O3 during the BST formation with melting method improve breakdown strength and energy-storage density significantly.

Single cite
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Zhang, W., Wang, J., Xue, S. et al. Effect of La2O3 additive on the dielectric properties of barium strontium titanate glass–ceramics. J Mater Sci: Mater Electron 25, 4145–4149 (2014). https://doi.org/10.1007/s10854-014-2141-6




References

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