chemistry

Hafnium compounds

Hafnium compounds is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Hafnium compounds rather than just read about it. In short: Hafnium compounds are compounds containing the element hafnium (Hf). Due to the lanthanide contraction, the ionic radius of hafnium(IV) (0.78 ångström) is almost the same as that of zirconium(IV) (0.79 angstroms).

Key takeaways

Hafnium compounds belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Hafnium compounds to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Hafnium compounds from memory before moving on to harder problems.

Reference excerpt

Hafnium compounds are compounds containing the element hafnium (Hf). Due to the lanthanide contraction, the ionic radius of hafnium(IV) (0.78 ångström) is almost the same as that of zirconium(IV) (0.79 angstroms). Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties. Hafnium and zirconium tend to occur together in nature and the similarity of their ionic radii makes their chemical separation rather difficult. Hafnium tends to form inorganic compounds in the oxidation state of +4. Halogens react with it to form hafnium tetrahalides. At higher temperatures, hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Some compounds of hafnium in lower oxidation states are known.

Halides Hafnium(IV) fluoride (HfF4) is a white crystalline powder. It has a monoclinic crystal structure, with space group C2/c (No.15), and lattice constants a = 1.17 nm, b = 0.986 nm and c = 0.764 nm. Hafnium(IV) chloride (HfCl4) is also a white crystalline powder, with a monoclinic structure. It can be prepared in many ways:

By the reaction of carbon tetrachloride and hafnium oxide at above 450 °C; HfO2 + 2 CCl4 → HfCl4 + 2 COCl2 Chlorination of a mixture of HfO2 and carbon above 600 °C using chlorine gas or sulfur monochloride: HfO2 + 2 Cl2 + C → HfCl4 + CO2 Chlorination of hafnium carbide above 250 °C. Hafnium(IV) bromide (HfBr4) is a colourless, diamagnetic moisture sensitive solid that sublimes in vacuum. It adopts a structure very similar to that of zirconium tetrabromide, featuring tetrahedral Hf centers, in contrast to the polymeric nature of hafnium tetrachloride. Hafnium(IV) iodide (HfI4) is a red-orange, moisture sensitive, sublimable solid that is produced by heating a mixture of hafnium with excess iodine. It is an intermediate in the crystal bar process for producing hafnium metal. In this compound, the hafnium centers adopt octahedral coordination geometry. Like most binary metal halides, the compound is a polymeric. It is one-dimensional polymer consisting of chains of edge-shared bioctahedral Hf2I8 subunits, similar to the motif adopted by HfCl4. The nonbridging iodide ligands have shorter bonds to Hf than the bridging iodide ligands. Hafnium(IV) chloride and hafnium(IV) iodide have some applications in the production and purification of hafnium metal. They are volatile solids with polymeric structures. These tetrachlorides are precursors to various organohafnium compounds such as hafnocene dichloride and tetrabenzylhafnium. Hafnium does form lower halides such as hafnium(III) iodide. Hafnium trihalides are strongly reducing compounds and as such do not have any aqueous chemistry.

Oxides

The white hafnium(IV) oxide (HfO2), also known as hafnium dioxide or hafnia, with a melting point of 2,812 °C and a boiling point of roughly 5,100 °C, is very similar to zirconia, but slightly more basic. It is an electrical insulator with a band gap of 5.3~5.7 eV. Hafnium(IV) oxide typically adopts the same structure as zirconia (ZrO2). Unlike TiO2, which features six-coordinate Ti in all phases, zirconia and hafnia consist of seven-coordinate metal centres. A variety of other crystalline phases have been experimentally observed, including cubic fluorite (Fm3m), tetragonal (P42/nmc), monoclinic (P21/c) and orthorhombic (Pbca and Pnma). It is also known that hafnia may adopt two other orthorhombic metastable phases (space group Pca21 and Pmn21) over a wide range of pressures and temperatures, presumably being the sources of the ferroelectricity observed in thin films of hafnia. Thin films of hafnium oxides deposited by atomic layer deposition are usually crystalline. Because semiconductor devices benefit from having amorphous films present, researchers have alloyed hafnium oxide with aluminum or silicon (forming hafnium silicates), which have a higher crystallization temperature than hafnium oxide.

Other compounds

Hafnium diboride belongs to the class of ultra-high temperature ceramics, a type of refractory ceramic composed of hafnium and boron. It has a melting temperature of about 3250 °C. It is an unusual ceramic, having relatively high thermal and electrical conductivities, properties it shares with isostructural titanium diboride and zirconium diboride. Nanocrystals of HfB2 with rose-like morphology were obtained combining HfO2 and NaBH4 at 700-900°C under argon flow:

HfO2 + 3NaBH4 → HfB2 + 2Na(g,l) + NaBO2 + 6H2(g) Hafnium carbide is the most refractory binary compound known, with a melting point over 3,890 °C, and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of 3,310 °C. This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures. The mixed carbide tantalum hafnium carbide (Ta4HfC5) possesses the highest melting point of any currently known compound, 4,263 K (3,990 °C; 7,214 °F). Recent supercomputer simulations suggest a hafnium alloy with a melting point of 4,400 K. Hafnium forms both a hafnium(III) and a hafnium(IV) nitride. Hafnium silicate (HfSiO4) is a silicate of hafnium, and it is a tetragonal crystal. Thin films of hafnium silicate and zirconium silicate grown by atomic layer deposition, chemical vapor deposition or MOCVD, can be used as a high-κ dielectric as a replacement for silicon dioxide in modern semiconductor devices. Hafnium(IV) nitrate (Hf(NO3)4) is the nitrate of hafnium(IV). It can be prepared by the reaction of hafnium tetrachloride and dinitrogen pentoxide. Hafnium disulfide is a layered dichalcogenide with the chemical formula of HfS2. A few atomic layers of this material can be exfoliated using the standard Scotch Tape technique (see graphene) and used for the fabrication of a field-effect transistor. High-yield synthesis of HfS2 has also been demonstrated using liquid phase exfoliation, resulting in the production of stable few-layer HfS2 flakes. Hafnium disulfide powder can be produced by reacting hydrogen sulfide and hafnium oxides at 500–1300 °C.

See also Titanium compounds Zirconium compounds Lutetium compounds Tantalum compounds Lanthanide contraction

References

Illustrations

Worked examples

Example 1 — a first encounter with Hafnium compounds

Start with the simplest possible case. Write down what Hafnium compounds claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hafnium compounds before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hafnium compounds ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hafnium compounds

In research: Hafnium compounds appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hafnium compounds in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hafnium compounds is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical compounds by element, Hafnium, Hafnium compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Hafnium compounds outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Hafnium compounds in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Hafnium compounds means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Hafnium compounds out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Hafnium compounds in simple terms?

Why does Hafnium compounds matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hafnium compounds?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hafnium compounds.