Introduction

 Lanthanum(III) oxide (La₂O₃) is a p-type semiconductor with the largest band gap among rare earth oxides (4.3 eV),  high dielectric constant (ε = 27) ^[1]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.^[2]

There are four known polymorphs for La₂O₃: C-type, A-type, H-type and X-type. The phase transitions depend on the temperature. ^[3]

C-type (Ia-3) is considered to be the structure with lowest energy and can transform to A-type (P-3m1) around 870K^[4]

H-type (P63/mmc) and X-type (Im-3m) are high-temperature phases of La₂O₃. The transformation from A-type to H-type occurs around 2300 K and can be further transformed to X-type around 2373K.^[5].

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.^[4]

Synthesis

Obtain amorphous La₂O₃ from La(OH)₃ and 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.^[4]

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) (Figure: Xueran Tao)

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.^[2]

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) (Figure: Xueran Tao)

 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^[5], 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) (Figure: Xueran Tao)

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^[5]. 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) (Figure: Xueran Tao)