{"id":672636,"date":"2026-11-02T12:18:11","date_gmt":"2026-11-02T12:18:11","guid":{"rendered":"https:\/\/www.newsbeep.com\/ie\/672636\/"},"modified":"2026-11-02T12:18:11","modified_gmt":"2026-11-02T12:18:11","slug":"inside-the-long-running-argument-over-bonding-in-phase-change-materials-research","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/ie\/672636\/","title":{"rendered":"Inside the long-running argument over bonding in phase change materials | Research"},"content":{"rendered":"

For more than a decade, Richard Dronskowski<\/a> and Matthias Wuttig<\/a>, both at RWTH Aachen University in Germany, have been battling about the bonding in chalcogenide phase change materials. They have helped develop \u2013 and become rooted to \u2013 contrasting theories as to what\u2019s going on at the atomic level. While the pair once worked together to understand these materials, countless heated arguments and published exchanges mean they\u2019re no longer on speaking terms. The few kilometres between their offices are nothing compared to the deep disagreement that separates the two scientists.<\/p>\n

One of the reasons why these researchers, as well as others in the UK, Spain, Belgium and South Korea, are so committed to understanding how atoms in these materials bond is to help explain their unusual properties. Phase change materials reversibly convert between an amorphous and crystalline phase in nanoseconds, prompted by a laser or an electrical pulse. Few materials undergo structural changes so quickly \u2013 just look at the amorphous silica found in windowpanes, which stays in the same phase for many years.<\/p>\n

The crystalline and amorphous phases in these materials also often have different refractive indexes. This offers a way to store information as 0s and 1s (like the binary code used by silicon chips in computers) based on the different directions of light passing through the material\u2019s different phases. Rewritable DVDs make use of this feature, for example, by using a laser to change the phase of part of the material on the disc\u2019s surface.<\/p>\n

\"Phase<\/p>\n

More recently, researchers have been looking at using phase change materials for neuromorphic computing, which saves, analyses and learns from real-time data in an analogous way to neurons in the brain. By tuning how crystalline the material is, researchers can create a continuum of states between fully amorphous and fully crystalline, allowing information to be represented as more than just 0s and 1s. Synthetic neurons made with these materials can use this to carry out multiple tasks at any given moment, enabling faster solutions to complex calculations.<\/p>\n

\"Richard<\/p>\n

\u2018It\u2019s obvious these materials defy our conventional wisdom of what\u2019s metal, what\u2019s covalent [and] what\u2019s ionic [bonding],\u2019 Wuttig says. He \u2018strongly believes\u2019 that there is a link between the chemical bonding, the material\u2019s band structure (the range of energy levels that electrons can occupy in solids) and the material\u2019s properties. \u2018If we manage to interlink the three, we have the best chance ever to [better design these materials],\u2019 he says. Dronskowski is on the same page. \u2018If you want to understand function, study structure,\u2019 he says, quoting Nobel laureate Francis Crick, who co-discovered DNA\u2019s double helix.<\/p>\n

However, the researchers are championing two different theories as to how the atoms in the crystalline phase of these materials form bonds. Dronskowski thinks that atoms in these materials form multicentre, electron-rich bonds, with orbital-based methods backing his theory.1 He has dubbed these bonds \u2018hypervalent\u2019. On the other hand, Wuttig has opted for electron density-based calculations that suggest that the bonding is electron deficient, with these \u2018metavalent\u2019 bonds sitting between metallic and covalent bonding.2 These theories are polar opposites, raising the question of which one, if either, gives the best view of the bonding in these materials.<\/p>\n

<\/p>\n

This is an ongoing active controversy<\/p>\n

<\/p>\n

The researchers also seem to disagree on when their theories started to drift apart. From Dronskowski\u2019s perspective, Wuttig previously was open to the hypervalent theory. But then \u2018he thought because the [properties of these materials are] exceptional, the bonding must also be exceptional, which is not true. And then this was dubbed metavalent,\u2019 says Dronskowski. However, Wuttig says that Dronskowski\u00a0was the one who \u2018pulled out\u2019 from the idea that the bonding was metavalent.<\/p>\n

\"Matthias<\/p>\n

Dronskowski and Wuttig are not alone in debating these materials. While many chemists and physicists have aligned with one of the two competing theories, others have proposed alternative bonding models or questioned the value in studying the bonding in these materials in the first place.<\/p>\n

Unusual structures<\/p>\n

Germanium telluride (GeTe) is one of the simplest examples of a chalcogenide phase change material. Chalcogens include tellurium, oxygen, sulfur and selenium. Other common phase change materials include antimony telluride (Sb2Te3) and germanium antimony telluride (Ge2Sb2Te5).<\/p>\n

In the crystal phase, these materials tend to adopt a distorted rock salt structure, where all atoms are octahedrally coordinated. Typically, this structure is expected for materials where atoms have a large electronegativity difference, like in sodium chloride. In phase change materials, however, germanium, tellurium and antimony have very similar electronegativity values, meaning that the amount of charge transfer between these atoms is low.<\/p>\n

Dronskowski and Wuttig are unanimous when it comes to suggesting that the bonding mode is likely why these materials adopt an unusual crystal structure. However, they have different opinions on how this happens.<\/p>\n

The puzzle is \u2018easily solved by the fact that you eventually realise there are too many electrons\u2019, suggests Dronskowski. To accompany these electrons, he says that the atoms form three-centre four-electron (3c-4e) bonds. While the excess electrons form an antibonding interaction between neighbouring atoms, they are net-nonbonding over three atoms, allowing the electrons to spread out over the material, Dronskowski explains. This weakens the overall interaction, making it easier to switch between the crystalline and amorphous phases.<\/p>\n

<\/p>\n

The whole focus on bonding is taking us away from how interesting these materials are<\/p>\n

<\/p>\n

In contrast, Wuttig thinks that the same bonds are in fact electron deficient, better described as two-centre one-electron (2c-1e) bonds \u2013 extending this leads to three-centre two-electron (3c-2e) bonds. The electrons in these bonds are also delocalised through the material.<\/p>\n

Multicentre bonding between atoms in the crystal phase of these materials is the common thread in each theory. Dronskowski explains that these bonds are linear as \u2018it\u2019s the p orbitals which overlap,\u2019 adding that \u2018this immediately explains the [distorted rock salt] structure.\u2019 As heavy atoms like tellurium and antimony also have large, diffuse orbitals, the atoms are easily polarised, allowing for more effective overlap between multiple atoms.<\/p>\n

Evidence for multicentre bonding comes from atom probe tomography experiments. This involves bombarding crystals with gaseous cations \u2018like using a machine gun against a concrete wall,\u2019 explains Dronskowski. Analysis reveals clusters of atoms hitting the detector, which Dronskowski says is indicative of multicentre bonding. Similar experiments with ionically bonded magnesium oxide, for example, instead generates single magnesium and oxide ions.<\/p>\n

However, such techniques only give enough information to infer the bonding in these materials. Studying the bonding between atoms instead relies on computational models, as it is challenging to study the electron density between atoms experimentally. And there are many approaches, which unhelpfully lead to different results. Dronskowski and Wuttig have used different computational methods to reach their respective conclusions \u2013 a problem that lies at the heart of the disagreement between these researchers, as well as others.<\/p>\n

Taking the atomic approach<\/p>\n

Dronskowski says these phase change materials break the octet rule \u2013 a rule of thumb that says an atom forms bonds to have a total of eight valence electrons. For example, in GeTe, Dronskowski says that germanium and tellurium donate all their valence electrons \u2013 four and six electrons, respectively, giving a total of 10 electrons per formula unit.<\/p>\n

He highlights that compounds such as gaseous xenon difluoride (XeF2) also break the octet rule, with eight electrons from the xenon and two bonding electrons from the fluorine. \u2018Therefore, nature needs to distribute the electrons in a three-centre bond,\u2019 he says. He thinks that the same can be said for phase change materials, with the two excess electrons shared between three atoms to create a 3c-4e bond.<\/p>\n

He has reached this conclusion by analysing the bonding in phase change materials \u2018by looking at the wave function\u2019 of the electrons. To do this, his team has come up with a computer program known as Lobster (local orbital basis suite towards electronic-structure reconstruction).3 \u2018We can take the entire electronic structure [of the material], project it down to atomic or molecular fragment orbitals\u2019 and then construct molecular orbital diagrams, he says.<\/p>\n

Dronskowski stresses the benefits of using an orbital approach by likening it to drawing a scheme that shows how the 1s orbitals in H2 interact, which leads to a bonding and antibonding orbital. \u2018If you try to look at [the bonding] from the density [of electrons], it becomes much more complicated,\u2019 he says. Electron density is always positive, he notes, which ignores the different atomic solutions and whether electrons in the wavefunction are bonding, antibonding or nonbonding.<\/p>\n

Dronskowski\u2019s analysis suggests that atoms in these materials form 3c-4e or hypervalent bonds. \u2018I don\u2019t have an idea why this is hard to swallow for some people,\u2019 he says. \u2018It has been observed decades before and it\u2019s an exciting discovery that you have the same bonding [as in other electron rich materials] also in phase change materials,\u2019 he says.<\/p>\n

Making the case for metavalency<\/p>\n

Wuttig disputes Dronskowski\u2019s theory, as he thinks that electron-rich hypervalent bonding \u2018cannot exist in solids\u2019. He explains that making 3c-4e bonds in three dimensions would require a total of 12 electrons per unit cell \u2013 two more electrons than the maximum number possible. He also highlights that the s orbitals of heavy atoms like tellurium are much lower in energy than the p electrons, meaning that electrons in these orbitals are not involved in bonding. \u2018Which electrons matter is hidden behind this [dispute],\u2019 Wuttig says.<\/p>\n

To analyse these materials, Wuttig has instead opted to use electron density-based calculations, which predict that two adjacent atoms share one electron, rather than two (2c-1e bonds). \u2018If you extend this \u2013 if you make a line \u2013 you have two electrons between three atoms,\u2019 he says. Wuttig notes that these 3c-2e bonds are the same as those in compounds such as diborane (B2H6). Nobel laureate William Lipscomb studied these structures using x-ray crystallography, which led to him winning the 1976 Nobel prize in chemistry<\/a>.<\/p>\n

Plotting the number of electrons shared versus electrons donated between atoms produces distinct islands of materials, such as covalent, metallic and ionic, says Wuttig. Chalcogenide phase change materials sit as a distinct group of their own, sandwiched between metallic and covalent materials, he argues. This has led Wuttig to describe this bonding as \u2018metavalent\u2019<\/a> \u2013 a portmanteau of metal and covalent.<\/p>\n

\u2018With this scheme we can easily explain why the bonds are so soft,\u2019 says Wuttig. He adds that his team used the theory to tailor the thermoelectric properties of some phase change materials. A European semiconductor manufacturer is currently working with Wuttig to build better high-frequency switches: \u2018We could find a material that was 10 times better than the best material this company had so far,\u2019 explains Wuttig.<\/p>\n

However, Stephen Elliott<\/a> at the University of Cambridge, UK, notes that \u2018if you take away the colour [from Wuttig\u2019s plot], it looks just the same,\u2019 adding that the metavalent region blends in with similar, covalently-bonded materials.<\/p>\n

\"2D<\/p>\n

Elliott believes in the hypervalent theory. He notes that Wuttig initially called his theory \u2018resonant bonding\u2019 but changed the name to avoid confusion with the bonding in molecules like benzene. \u2018[Wuttig] is still claiming that this metavalent bonding is a previously unknown form of bonding,\u2019 says Elliott. \u2018That\u2019s quite a big statement, frankly.\u2019<\/p>\n

Elliott notes that even though hypervalent bonding \u2018hasn\u2019t been invoked hardly at all in solid state extended materials, particularly for glasses and crystals, it is [already] well known\u2019.<\/p>\n

Yet, Wuttig would like those in the hypervalent camp to \u2018please do their homework\u2019 and better outline how the s electrons are in involved in 3c-4e bonding. Jean-Yves Raty<\/a> at the University of Li\u00e8ge in Belgium supports Wuttig\u2019s electron-deficient view of the bonding. \u2018What we have done is correct, the only missing link [is] the driving force behind this very peculiar type of bonding.\u2019<\/p>\n

Finding common ground<\/p>\n

Francisco Javier Manj\u00f3n Herrera<\/a> at the Technical University of Valencia in Spain was drawn in to the discussion about bonding in these materials after being asked to review one of Wuttig\u2019s studies. It focused on germanium selenide (GeSe), which is isoelectronic to pure arsenic \u2013 a material that Manj\u00f3n Herrera has previously studied.<\/p>\n

\u2018I started paying attention to what Matthias Wuttig and co-workers were doing,\u2019 he says. At a similar time, he came across work by those in the hypervalent camp, who had a completely different theory. \u2018So, I said \u201cwait, which one of the two teams is right?\u201d,\u2019 Manj\u00f3n Herrera says.<\/p>\n

After carrying out his own computational calculations on arsenic, Manj\u00f3n Herrera realised that the simple cubic phase of arsenic and the rock salt structure of phase materials both have electron-deficient multicentre bonds. He has now come up with his own theory as to how bonds may form, which he dubs the unified theory of multicentre bonding (UTMB).4<\/p>\n

He outlines how the UTMB theory works by describing how an applied pressure first brings atoms closer together. This allows a lone atom in another layer of the material to interact with an existing covalent bond, forming a multicentre bond. If the atom donates its lone pair of electrons, this leads to an electron-rich muti-centre bond, as vouched for by Dronskowski. However, in his theory, \u2018you just transfer an electron from the covalent bond to the new bond\u2019, thereby creating an electron-deficient version. He compares this to agostic interactions<\/a>\u00a0in organometallic chemistry, where the hydrogen atom of a C\u2013H bond, for example, interacts with a metal centre. Such interactions can also be described as a 3c-2e bond.<\/p>\n

To back the UTMB theory, Manj\u00f3n Herrera and his team have analysed how the bond distances between intra- and interlayer arsenic atoms changes with pressure. Initially both distances decrease with pressure, as would be expected. However, at around 15GPa, the shorter interlayer distance increases slightly. \u2018This increase is related to the decrease of [the bond\u2019s] vibrational frequency,\u2019 Manj\u00f3n Herrera says, adding that this shows \u2018the loss of charge of this covalent bond\u2019. After 25GPa, there is only one measurable bond distance, linked to a multicentre bond, he explains.<\/p>\n

Manj\u00f3n Herrera says that some chemists in the metavalent camp have now \u2018started to recognise our model\u2019. He describes a study that attributes the bond order of 1 between atoms to both metavalent and hiw own bonding models,5 adding that this makes the metavalent description \u2018useless\u2019. Equally, he notes that the \u2018hypervalent\u2019 term is not needed as it is \u2018just an electron-rich multicentre bond.\u2019 \u2018There is no need to name it otherwise, specially considering that there is no violation of the octet rule due to a larger valency than expected.\u2019<\/p>\n

Similarly, Robert Jones<\/a> at Forschungszentrum J\u00fclich in Germany thinks that the discussion about bonding in these materials is just \u2018a big distraction\u2019. \u2018The whole focus on bonding is taking us away from how interesting these materials are and focusing on something which we can only argue about \u2013 not something that we can measure,\u2019 he says. Jones is a strong proponent for the electron-rich theory of bonding and has been involved in the disagreement about the bonding in these materials for some time. \u2018I think that to argue about labels is probably not very helpful and the question is why did this discussion begin at all,\u2019 he says.<\/p>\n

\u2018This connection between properties and bonding is something that was made and this is something that I dispute,\u2019 Jones says. \u2018There is a correlation, but this doesn\u2019t mean causation.\u2019 Jones also thinks that there is no difference between the bonding in the crystalline and amorphous phases of these materials. For him, focusing on the materials\u2019 structures (which is directly measurable) is much better than understanding how atoms bond (which is not).<\/p>\n

\u2018I can\u2019t tell people how they\u2019re going to get their inspiration\u2026 maybe they can think about things in odd ways,\u2019 Jones says. \u2018[Others] have a picture of what\u2019s going on, which is different from mine and probably in details it\u2019s not correct, but nonetheless they can get to useful conclusions,\u2019 he says. However, Elliott thinks that knowing the bonding type could help scientists computationally design new materials by adjusting the type and amount of each element.<\/p>\n

An ongoing discussion<\/p>\n

Challenging ideas is part of the scientific process. \u2018In science, there is no hard and fast right answer,\u2019 says Elliott. \u2018You try to explain all the facts as they are at a particular moment in time and, of course, what we say could be proved wrong.\u2019 He also notes that it\u2019s necessary to acknowledge contrasting theories, particularly in scientific papers. Elliott thinks that he and others in the hypervalent camp do this, while those in the metavalent camp only celebrate their own theory. \u2018This is an ongoing, active controversy. It has not been solved yet,\u2019 says Elliott, although he still backs the hypervalent theory.<\/p>\n

For now, the two camps remain divided on whether the bonding between atoms in these materials is electron rich or electron deficient. \u2018You can imagine if there are two professors from the same university that even in the past have published together \u2013 and now they don\u2019t even talk to each other anymore \u2013 that this is emotionally draining,\u2019 says Wuttig. \u2018I wish we would just simply sit together and discuss.\u2019 Until that happens, the bond between these scientists will remain broken.<\/p>\n","protected":false},"excerpt":{"rendered":"For more than a decade, Richard Dronskowski and Matthias Wuttig, both at RWTH Aachen University in Germany, have…\n","protected":false},"author":2,"featured_media":672637,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[61,60,82],"class_list":["post-672636","post","type-post","status-publish","format-standard","has-post-thumbnail","category-science","tag-ie","tag-ireland","tag-science"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/posts\/672636","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/comments?post=672636"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/posts\/672636\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/media\/672637"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/media?parent=672636"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/categories?post=672636"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/ie\/wp-json\/wp\/v2\/tags?post=672636"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}