Introduction and properties

PtO₂ exists in three crystalline phases, α-PtO₂, β-PtO₂ and a theoretical phase β’-PtO₂. Unlike α-PtO₂ and other Group 8 metal dioxides, β-PtO₂ has an orthorhombic CaCl₂ -structure. β-PtO₂ is a black, highly insoluble solid, and it is the most stable out of the PtO₂ -variations under standard temperature and pressure. ^[1][2]Stability of the peculiar orthorhombic crystal structure of β-PtO₂ is due to strong hybridization between 2p-orbital of oxygen and 5d-orbital of platinum. When the densities of states curves are drawn for both β-PtO₂ (CaCl₂) and β’-PtO₂ (rutile), an absence of a peak at the fermi level can be observed for β-PtO₂. This results in a pseudo-gap in the fermi level, possibly causing interesting properties. ^[3]However, there is currently very little experimental data on β-PtO₂. Some physical properties of β-PtO₂ are listed in table 1.

Table 1. Properties of β-PtO₂ ^[4]

Structure

β-PtO₂ crystallizes in the orthorhombic CaCl₂ -structure, which is similar to a tetragonal rutile structure, but with small distortions in the directions of a- and b- axes. The tetragonal rutile structure could in theory be obtained when β-PtO₂ is heated over 1240 K, resulting in a variation called β’-PtO₂. ^[1][6]In β-PtO₂, the each Pt atom is coordinated to six O atoms and each O atom to three Pt atoms. ^[3] The structural information of β-PtO₂ is listed in table 2, and the structure is illustrated in figures 1 and 3. A comparison between orthorombic CaCl₂ structure and tetragonal rutile structure is shown in figure 2. The simulated XRD powder pattern is shown in figure 4. 

Table 2.  Structural information of β-PtO₂ ^[4]

Figure 1. Structure of β-PtO₂, on the left ball-stick model (ionic radii), on the right polyhedral model (atomic radii). Red balls represent oxygen, gray balls represent platinum. (Figure: Sini Suurnäkki)

Figure 2. Comparison between CaCl₂- and rutile-structure. (Figure: Sini Suurnäkki)

Figure 3. Structure of β-PtO₂ viewed from a-, b-, and c- axes, respectively. Red balls represent oxygen, gray balls represent platinum. (Figure: Sini Suurnäkki)

Figure 4. XRD powder pattern of β-PtO₂. (Figure: Sini Suurnäkki)

Synthesis

Single crystals can be obtained via high pressure synthesis. Before inventing high pressure synthesis, the precise crystal structure β-PtO₂ was unknown, because it could only be synthesized in powder form. β-PtO₂ crystals have been synthesized in a temperature of 1500 °C and pressure of 4 kbar, from Pt – KClO₃ -mixtures.^[4]

References